hoobiedoobie - for most people, 6x15mg would be barely noticeable, if that. If you had a strong experience with that little, perhaps you're one of the people born without the enzyme that normally metabolises DXM? At any rate, that's not a common experience, so I wouldn't read too much into it for what your friend will have.

LSD flashbacks tend to occur only when you take fairly high doses by today's standards. While a hit of acid might have been enough to trip face in the 60's, you need at least 3 or 4 average tabs nowadays to get there. I've heard of a couple people having flashbacks that were problematic, but most people don't get them without heavy use, and those that do usually don't mind them. Your friend should do some research on Erowid if he's really concerned.