Your friend is misinformed. You can IN THEORY get A+ weed from bagseed, but in reality that is usually far from the truth...most bagseed comes from imported weed (IBL sativas that will never finish in a non-tropical climate, yield very little, and may be of mediocre potency). The second most common reason to find a bagseed is that the grower had a hermaphrodite plant that pollinated another plant to make the seed. In this case the plant you grow form the bagseed will (almost certainly) go hermy on you.
The seed matters. Remember, phenotype = genotype (seed) + environment. If you give a bagseed the perfect grow conditions, you could get good weed off it, but you would be much better off just investing in a pack of good seeds or getting some clones. It's very discouraging to pamper a plant for three months only to find out that it's garbage or a hermy...