First off, ask if it was an emergency, or something that couldn't be postponed. Sometimes doctors think something is urgent enough to be seen right away, or need to give test results in person. She should have called you if she was called away, of course, but if she didn't have any other choice, maybe she gets some slack.
Second, ask if the boy is on her license as a child care provider. If he has care of your child, he needs to be finger printed, CPR certified, and have a TB test. It varies by area, of course, but she could get into plenty of trouble for leaving someone else's kid with her son.
Third, ask if anyone else has been in the house while she's away.
(HINT: If she's done it once, she's done it before. If she doesn't admit to other incidences - she's lying!)
I'd make my decisions on her answers.
If you decide to leave your daughter in her care, make it CLEAR that you want to be notified when she has appointments that take her out of the home.