A few thoughts. First, maybe your friend over watered while you were away. Second, if you're trying to flush, maybe let things dry out pretty severely for a few times.
To expand, following strict schedules can be challenging in organic grows, specifically as flower progresses. A flowering plant requires different levels of water; I'm sure you've seen that; it may not be a gradual progression from watering levels in vegging, in early flower and in mid to late flower. We think of that process in a linear way because that's how we see it: veg, early flower, mid flower, and late flower. But, watering levels change according to the stage in life cycle.
Did they change so much in the four days that you were gone that your friend over watered even though he added exactly the scheduled amount? Probably not. Is it possible he over watered? Was the water already measured out for him?
As a side note, it's good to keep a steady level of moisture. Thick layers of mulch help with this. But, if you're drying out in a 24 hour period and you can't miss a watering by a 4 hour window, I think you outgrew your pot. You can talk about autos or other things to help in the future. But for now, I think you should create a way to be able to keep your pots wet for at least 36 hours, maybe 48. If you are putting 20 gallons of water through that pot, at least 85% is washing through. Some would see this a problem, because it causes mobile nutrients to be washed through, (I've heard; just imagine what is in the brownish stuff draining from your pot). For the appropriate amount to water, I've seen ranges from 10% - 15% of soil volume. That'd be about 3 gallons. 21.5 gallons is a lot. It would flush, but that's flushing valuable nutrients out of the soil, whereas I think you're goal would more quickly met by killing the micro-herd.
I think you said it's in a 25 gallon pot; so you should be able to lift this. I'd get a huge saucer (whatever they're called in the grow world), maybe 2 or 3 feet, whatever size your pot, and create an outdoor sips pot. I'd get some panda film (plastic of any sort would work); coco coir or peat moss, (haven't tried the later); and pumice, lava rock, (really any rock like thing with more physical structure than powders) or a large stand. I've used the large stand, packing the legs of the stand with coco coir, with a 10 gallon pot; I currently have a 20 and 25 gallon smart pots in saucers with stands for worms, but the same idea applies.
Use either a rock/pumice or a plastic stand to support the pot above a reservoir in which water will sit. If using pumice, pack equal to half measure coco/peat to pumice. Try to have at least a 3-4 inch layer of rocks. I've never tried an all pumice base; I think it may actually transfer water too quickly
The best I've ever seen a plant grow is in the following condition. Your 25 gallon pot is simply scaled up. You didn't mention, but is it a smart/cloth pot? It appears so, anyways.
I used a 10 gallon smart pot in an appropriately sized saucer, sitting atop a plastic stand, whose legs are packed with coco-coir. Initially, all 6 legs were packed. The pot became too damp; I removed the pot and removed the coco in 3 of the 6 legs. The roots may not now grow down into the coco-coir; they will given enough time. But the key is that water will diffuse from the reservoir, through the legs of the stand that are packed with coco, to the bottom of the pot.
You've perfectly situated to try this outdoors. Humidity becomes an issue with this system indoors. (It shouldn't be a problem for you.) So, I wrapped the bottom half of the plan and the saucer in a skirt of panda film. Being indoors, this sufficiently reduced the impact of having 3 gallons sitting in a saucer in a tent. Roots blew out of the lower portion of the skirt; the upper portion being air pruned. Either in or out doors, I think this is important to prevent evaporation of water prior to use.
Concerning flushing, I'd just let things dry out. If, in biological systems, nutrients are derived from micro-herds in the soil, severely drying out that micro-herd has implications for the immediate and long term nutrient cycling process. The chemistry and biology is a bit beyond my domain. In hydro, you feed chems and flushing is when you stop feeding chems and feed only water. Drying out the micro-herd won't end all life. But if you want to flush, and flushing is using up the available nutrients, I think this might be the only way to mimic that process in nature.
It's a bit of back woods logic. I use 3, 7 and 10 gallon pots. All three have approximately the same beginning soil; in the intervening few years, all have received the exact same top-dresses and mulches. The three gallon, being that it dries out so much more quickly, consistently appears more flushed than the 7 gallon and usually the 10 gallon. This run is an aberration, where the 10 gallon has some gnarly fading. But, some of my pots are 2-3 years old in less than 10 gallons. So, it's entirely possible that it's just severely depleted soil. I'm planning to repot with my own earth worm compost this time, plus all the other amendments one should consider. I'd get away from perlite. Use pumice, rice hull, biochar, etc.
If I seem long-winded, it is because I owe my currently knowledge to rollitup. Each and every one of you beautiful bastards.
Your buds are clearly fantastic. That's something I didn't mention. I think people overthink soil sometimes. The use or discontinued use of specific top-dresses isn't likely to have serious immediate impacts on your plant. It's possible you experienced lockout if adding too much, but I don't know that that's what happened; and because it's a biological system, stopping adding something isn't likely to have immediate consequences. Discontinuing the use of things like aloe vera, enzymes or sugers (molasses) might make a plant less great, but I doubt it would severely impact a healthy, thriving plant. Many people do water only from seed to harvest.
I don't know what you should do. Do you have a few weeks/a month left? Do you have a USB microscope, and are you taking pictures of your leaves to find amber/clear ratios?
I doubt many have continued this far. The question to ask, of outdoor growers, is does it look like I'll easily have enough life in these leaves to finish? If so, and if you want to test the flushing theory, I'd let it dry out severely, and maybe defoliate a bit to let light all the way into your canopy. If you have perfect penetration, don't worry.
