Lost and found?

Mira has a problem. She has a 'friend' who is troubling her in small little irritating ways. Her things go missing or is messed up. For instance, her footwear go missing from where she left at church or elsewhere and is found after searching high and low in some other corner of the premises. Her watch or her ear rings go missing _ I dont think the person's intention is to steal but just to misplace them where she can't locate them. Her brand new kajol is used to scribble on the floor or her marker pen to scribble on the door. The worst was an obscene message going from her phone (which they take when we are not with them so that they can reach us in an emergency)to her dear friend. Of course we have no proof to nail the culprit save for the fact that the said person is with them when it all happens.
Mere suspicion is not enough to take it up with the parents. It will also spoil relations as most parents think their children are special and can do no wrong. Parents will rather believe their own kid than outsiders.
I assume this is a behaviour problem -jealousy towards a peer (girls will trouble girls only), strict parenting which makes her do forbidden deeds outside her domain and at the same time the confidence that she can fib well enough to convince the disciplinarian at home.
But what do I do if I'm not in a position to minimise interaction with the person?