Maybe I'm just too stoned to really care but the down stem for my pipe was only $25. Granted $25 is not exactly a small chunk of change in this economy, but I still don't think I'd hold a friend to it unless they absolutely insisted. I have 4 different down-stems for my bong, two 19mm and two 14mm down converters. I figure if one breaks we can keep smoking. If he broke the whole pipe, ya I'd hold him(or her) to that since it cost me $150.
I would try and find a shop that has the same HBG you have and match the length exactly, beyond that I personally like diffuser down stems. My pipe has 18 diffusers in 3 perc trees, and all of my downstems are diffused as well. I'm not a brand person, if it looks sturdy and the glass is solid I'm good with it. I've seen just as many high quality pieces and bong parts for $10 - $30 as I've seen $100+ parts that are crap.