I play with a serious phobia of deep water, and have no small amount of trouble in the water zones, so I understand where your friend’s coming from. This is just off the top of my head, so there may be more, but I think most are avoidable.
There is the spider guild rush, the spiders blocking the passage to Grenth, a couple of jumping puzzles that have spiders in or near them, and a small handful of hearts that deal with spiders, but beyond that, you can avoid them (and if your friend is not totally committed to 100% map completion, they can skip those hearts/JPs). Spiders are somewhat avoidable in GW2, though. It’s highly exploitable, and I’ve seen something like that go very, very bad in another game I used to play.
A texture mod that actually changes models would probably be something ANet would ban for.