"I'll have your son on Friday while you're in the hospital"
I don't really understand your complaint. I thought she did have your son on Friday!
Did she not?
I thought she had your son on Friday and returned with him at 2.30pm on Saturday.
So maybe three hours or so earlier than you expected, given that she'd said "Saturday evening", but a good enough 'practice run' I would have thought, and presumably (had you not yet been in) she would not have left him alone on the doorstep with a label fastened to the front of his top!
She had him on Friday night, she took him to the shops, got him ice cream, bought him a toy, took him to the park, and so they had their practice run!
"her idea but I will be finding somebody/something else."
You sound rather unreasonable to me, OP.