Let me get this straight - you're letting her live in your property, rent free, and she's accusing you of trying to sabotage her life?
In your situation, I would phone first contact a property lawyer in the area of your house, to see what your options are for getting rid of your
friends parasites. I say this because you may not simply be able to kick them out, depending on the local laws about residency and eviction.
Then I would phone Mark (not Rien) and lay things out plainly. You were happy to do them a favour by having them house-sit rent free. Rien's reactions recently have been totally appalling, and you feel like you've been slapped in the face and stabbed in the back in return for your attempts to help them. Quote texts if necessary. You're sorry it's gotten to this point, but you're not comfortable having them living on your property any more. Then, depending on your talk with the lawyer, lay out your plans for what happens next, which will involve them moving out in the near future.
Yes, this will probably tank the friendship, at least in the short term. But if Mark is marrying the crazy, your friendship is probably not going to survive anyways, unless you're willing to re-arrange your life and finances to make things happy and easy for Rien. And, as others have pointed out, you're risking the resale value of your house and possible litigation if Rien goes further, and sabotages the place in revenge.
At this point, I think accepting an apology would be find from a friendship point of view, but not from a managing your property point of view. Keep all copies of correspondence, including texts, in case things escalate and you need to involve the police.