Well, you know not to trust your fiancée's judgment in regards to her friends now. I'd try to see if you can do anything legally or via the justice system to recover the money or at least have them face charges.

From your description, I am not sure they are friends. And I am totally suggest a police/legal solution here.

Sounds like a low-key wedding. Also, ever heard of a bank?

Hang out with wrong crowd

Give them one chance to give back the money and pay the rent that’s due, otherwise sue then for the rent and contact the police about the theft. Problem solved. Unless they’re flat broke of course.

Police report. Not only did they steal from you, they didn't pay their rent. Small claims court for the rent though. Were they on a/your lease, or were they just handing you cash to live there? Get all documentation you can and again, go to the police.

Honestly, the time you spent writing this FML should have been spent filing a police report. They should be charged with grand larceny.

Light the torches! Sharpen the pitchforks! We're going on a hunt!

If they stole money report them to the police. Why keep that amount of money in the house anyway? A bank is safer.

Charge them with theft!