Similar situation many years ago. In my 20's I was dating a girl who only liked to see the good in people. A couple of young women in a nearby apartment started visiting her. After meeting them, I told my girlfriend she shouldn't trust them because they were rather sleazy, they had a druggie appearance & I didn't like the way they would wander around her apartment as if they were looking for valuables. She criticized my negative attitude & said I shouldn't judge people. Since we often argued about this, I said, "OK, sometimes learning the hard way is more effective."
After a few more visits from the "Sleaze Twins," my girlfriend called me & was very upset that a pair of gold earrings I gave her were gone.
She wanted to call the police, but I told her to forget it; she would have to prove that one of them stole the earrings & proving it would be difficult because they were not going to admit it & say, "Yeah, we stole the earrings because we needed drug money."
Since I gave her the receipt for the earrings in case she wanted to exchange them for something else, she knew the earrings cost $100.00 & she felt terrible, but I told her it was a cheap price for a valuable lesson.