In my opinion...
Would it be werid to offer to change an ABDL friends diaper?
I don't think it's weird at all if you share the same interest.
If you really want to I would say go for it, as long as your buddy can decide 'yes/no/another day if that would be okay' -that should be okay to offer.
Would you accept?
Depends on the person, if my best friend would ask I might say yes, but generally I would decline such an offer because I think the most friends of mine wouldn't ask for the right reason (for example...they might think they have to, because I could think - 'you being ABDL is okay for me' would not be enough for me)
Sincerely,
Sabu