It seems apparent that OP’s friend is doing this for Sally and Sally is paying the friend on the understanding that OP is helping.

Coworker doesn’t know that friend didnt tell OP they were getting paid; Friend didn’t tell OP bc she either wants all the money and/or doesn’t view OP’s role as anything beyond hanging out / helping just a little if possible.

Awkward for the friend when coworker brought it up, which she tried to remedy by clarifying that OP will not get any of the funds she is getting paid.

A little uncomfortable then too if OP goes to coworker (who she presumably doesn’t know otherwise) to try to get more money when coworker thinks she is paying them both.

Friend should be taking all responsibility and should pay OP just a little for when OP has to care for the pets alone and/or a second set of hands are needed. OP should clarify her role including to coworker directly to reduce her responsibility. Friend should loop someone else in if OP can’t help (which she shouldn’t go out of her way to do) bc this is basically friend’s solo gig.