Furious at this parent!!

[TinyTwigs]

I definitely get both sides. I would be livid if that ever happened to me but at the same time it's not your place or business to tell someone how to treat/discipline their child. If she took him out for ice cream then so be it, its not your place to get mad or to even stress out about something like that.
Friends or not do you really think she's going to change her parenting style for you? Clearly not bc if she did her sons behaviour would have changed and it clearly hasn't. Your biggest mistake was keeping him on and not terminating after everything you listed he did.
He sounds like a little punk and I would have kicked his ass out after the shit smears on the walls! That isn't typical temper tantrum behaviour that was extreme brat behaviour and would I never put up with that. I would have billed the parents for the cleaning lady to come disinfect my walls. I don't put up with behaviour like that from my own kids there's no way I would do it for strangers.
You're stressing yourself over something that you can't change bc even after you this something his new daycare provider or teacher will have to deal with it's not going to change bc the parents don't seem to care.
Forget about it and focus on the positive you have going for you!