I was the tenant in this situation. I was the one who moved out. It was messy, as you will soon figure out. We had it all arranged, I was staying with a friend and would be back on the weekend and the following weekend to get the rest of my shit, because work. I agreed to pay my half the rent and the utilities until I was taken off the lease. We agreed this would happen by the end of the month.

The ex entered into the false belief that he could be pettier than me. He wanted to show his ass and mistakenly thought my ass wasn't larger (figuratively and literally). He was buddy buddy with the manager, but he waited until the manager's day off and got the dumbass assistant manager to change the locks.

I got a court order, they gave me a >new< key (meaning ex had to pay to get the locks changed AGAIN after I got off the lease) got my shit, got signed off the lease. But there was still an active lawsuit against the landlord. And then the landlord didn't show up in court, I was counting on that. Yes it was an asshole move on my part but fuck that property management company and fuck my ex and fuck his friendship with the on site manager and fuck all our neighbors who he got in the split, and fuck their buddy buddy with the apartment manager. Yup, it got very very awkward with the apartment manager and my ex, and the manager didn't even take sides. Fuck everyone.

So anyhoo, just as I was counting on, the landlord didn't show up in court and I won a default judgement, with interest, and a lien against the property.

I got a lien against the property, and didn't tell the landlord. I waited six months before contacting to collect. Because the landlord could have appealed and would have had everything overturned. From the judge's perspective, waiting six months to try and appeal is very disrespectful to the bench, and the judge basically told them to fuck off.

Then the landlord (note that this is six months + later and the ex is six months + out of the picture) still didn't pay. And several years later, they tried to go condo. But they couldn't sell because I had a lien on the property. With interest.

They gave me a cashier's check for the balance plus interest plus the court costs, as per the lawsuit i won by default because they didn't bother to show up and didn't bother to immediately appeal.

I made a nice amount of cash. It was extremely petty and fuck that property management company and my ex found out and now I'm a pettier asshole than he is so fuck him too. Fuck everyone all around.

And that's why you shouldn't get involved. The boyfriend needs to get a court order to get her off the lease and probably not to ever contact him, because as long as bills are current and she is on the lease, she has the right to come and go as she pleases. And if you change the locks, she will be furious and you'll be easier to take it out on than her.