M.F.
Sounds like you need to explain to your daughter that families come in all shapes and forms, not all have a mom and a dad, and that's ok.
It also sounds like you need to be a little less judgy of the mom. The woman has had 3 boyfriends in 4 years, not 4 months. If you don't want her to have visitors of any kind over when your daughter is there, then only have the girls play at your house, but make sure you don't have anyone visit when her daughter is there, not makes, females, repair persons, etc.
As for the candy play, she's a kid and she's venting. When mom and her boyfriend stop getting along and the relationship ends, it's over completely for the daughter, even if she's grown to care for the boyfriend also. She's acting that finality out. It's only unhealthy if she forgets to brush after eating the candy.