considering their size, for taking pictures of theraphosid spiders, the normal kit lens is more than enough.
I never found that they have any features that are worth doing extreme macros on, unlike insects, they have very poorly developed eyes. I always find a picture of the full spider on dorsal view is the nicest.
Of course, once you get into it, your BIL might start acquiring some spiderlings, and these would require a set-up that would at least give you 1:1 reproduction ratio... and for some species, it is not even enough as the babies are really tiny.
But for adults, it would be more than enough, aside from some dwarf genera like Cyriocosmus for example, which some species are quite tiny at adult size.
Here is a pair of Cyriocosmus elegans mating in my hands!