First of all, there is no amount of money worth being treated like that in your own home!!
She is sooo just trying to be in control. I would NOT email the dad, just keep as much infor between you and the person doing thje drop off or pick ups.
If they dont bring a clean change of clothing then oh well, if the kid gets dirty or wet then happily hand him off to mom or dad and tell them that the next day, if no clean clothes are brought with him, then you will NOt be taking the kid. and MEAN it.

I use the spare set ONLY when kids get super dirty from mud or food spills or super wet from the backyard water barrel or whatever else might happen.
I sent a girl home in a white frippy lace skirt COVERED in sand. It was brown all over. Oh well, I think by not changing her before she went in the sandbox, and allowing her to get dirty, it was sending a message to the parents, that kids should come in PLAY CLOTHES! I dont feel bad one bit about them getting dirty from playing!!
Dont fret about every little spill and food that is on their shirts, s**t happens. they can suck it up.
LOL!!
Let us know how tmrw goes! maybe it should be a spaghetti lunch and fingerpainting day if they dont bring a change of clothes!! hehe