Well, I have a happy update!
Firstly, as some other posters surmised, there is more detail behind all of this. Most important I think is that Mark is a very old & dear friend of mine; one of my best friends, actually. I knew him long before Rien did, and was actually the first person he introduced his "new" girlfriend to.
Secondly (and this was news to me), Mark & Rien had a series of disasters over the last couple of weeks, all wedding-related
- their photographer backed out (she was doing the wedding for free, as a gift)
- another bridesmaid found out she leaves for basic training 2 days before the wedding
- a third bridesmaid has been behaving... terribly (I'll leave it at that)
- their families have gone wedding-crazy
So there were a lot of other things affecting our situation with them.
And now to the update!
The Hawk & Mark had a talk (email), and then Mark and I had a very long conversation; Mark had been unaware of Rien's rantings, and apologised profusely to both The Hawk and me. He then sat down with Rien, and somehow set her straight.
Rien called me, and we talked, and cried, and talked more. She is one of my closest friends, so being on the outs with her was very painful.
She and I hashed out a deal that we will leave The Hawk & Mark to discuss house-business, and she and I will focus on our friendship and the wedding.
I am happy with this outcome, as is The Hawk; the four of us will be able to work together like a team - landlord/tenant - until they move, and then we can go back to just being friends
Thank you all for your input; I'm grateful that I didn't explode at my friends, and thankful that I got so much insight into what was going on!