Update:
We still haven't fixed it entirely,
1. clearing the cache, whether through manually deleting the folder as I described above, or through the steam settings ui, didn't work
2. changing his DNS to Google's didn't do anything
3. adding steam to Windows Defender exclusions list made the background of steam show up black, but the majority of formatting is still not working
4. we made sure that Steam was allowed under his firewall rules too.
at this point I have no other ideas and just wanna chalk it up to his isp (xfinity) being ass about this somehow
if anyone else wants to try their hand at googling this problem, I was looking along the lines of "steam css broken" and stuff