My guess is your friend is lying. Acid doesn't do that... Not sure why people say this. When I was younger (16 or so until I was about 25 I guess) I took a LOT of acid. By a lot I mean quite regularly and occasionally very high doses (100~ hits or so at a time). Seriously, yes, things were crazy. Everything was very strange, the visuals/auditory stuff was crazy, but I never in my entire life saw something that wasn't there...except one time. That one time involved about 1/2oz of psilo shrooms and about 15 hits of some decent gels. My assumption (as I've never had shrooms sans acid) was either an interaction or the psilocybin itself. These were hallucinations and I knew they weren't real (hallucinations are not assumed to be real, those would be delusions). I was watching law & order and everyone had large wooden tableware instead of heads (like those big forks / spoons old people have in their houses). The neat thing about that is that they followed the correct perspective of things, if looking left, the spoon's spoon end was facing that direction etc. Second, I saw full color hieroglyphics on the wall for about 10 minutes. That was odd. Aside from this case, the only time on acid I've seen something I thought was there but wasn't was the shadow shifting in dark corners / rooms etc. Since everything in in motion what is normally a still shadow will shift and look as if someone walked in front of the light, usually making turn around to see who it was, then I realize, oh, the shadow is just appearing to move around.

Don't let people tell you they see X when on acid. Things distort, move around, covered in strange patterns (like when I look at the carpet I see shapes that appear to be a contrasted / silhouetted mandelas, like this picture, but just a pattern in the dark/light of the carpeting..nothing distinct) and otherwise look rather odd. Mentally you're pretty off center, things seem to have relevance when they don't. Connections are made between two events that have no real connection, etc. But again, no magic spiders doing any such thing. I hate this misconception a lot, because I hear it all the time, usually it turns out they've never even done acid, in my experience (ie. being around them when they actually did and them expressing the fact).