Advice based on my personal experience: she needs to have a plan - a script really - for what she will do if/when (most likely when!) he calls up wanting to come home.
Her situation sounds so VERY close to what mine was. My now-X called two days after he walked out wanting to come home. Fortunately, I KNEW he was going to do that, and was prepared. I had decided exactly what I would say to him when he called, and forced myself to stick to that no matter what. And he did the whole gamut - hinted at suicide, first very subtly and then almost blatantly stating it. He cried. He begged. My answer was that I was willing to at least give R a try, but that he couldn't come home for at least a year, and that there were no guarantees after that. And that we had to go to counseling. And that he (obviously, to me, but I figured I needed to state it) couldn't see the OW anymore. (Note: I guess I should have stipulated that included NOT living with her either!)
The reason I often stress to people on here about making a plan for what to do and say when the WS calls wanting to come home is that it gets emotional. It's way to easy to give in to a bad situation because the alternative (D when you didn't see it coming and didn't want it) seems impossible. Better to make a reasonable plan for how to handle the situation before it happens and emotions start overriding better judgement. The only hard part then is forcing oneself to stick to the plan!
In my case, I knew he would call begging to come home. I knew it was a matter of when he called, not if. And it only took two days. And if I hadn't made the plan in advance, I probably would have ended up letting him come back.
What I found out later was - he only called wanting to come home because they had fought and she had kicked him out. As soon as they patched it up, he was living with her again - even though he spent 2 1/2 months covering that up and pretending to R with me. He spent the next year or so bouncing between her place and his parents' house every few days because she was kicking him out 2 or 3 times a week.
My plan kept the front door of my house from becoming a revolving door.
The one other thing it did.... A few weeks before he walked out, we went to visit my family for Christmas several states away. He tried like crazy to get me to go without him. I refused - said either we both went or neither would go. (Actually, I told him he could go and I would stay... considering my whole family hated him, he didn't really think much of that suggestion. )
I found out later that he had planned to leave while I was out of town at Christmas. I have no doubt I would have come home to a house that had been completely cleaned out of anything of any value. He ended up leaving with barely an overnight bag instead.
And I have no doubt that if I had let him come home after that, he would have waited for a good opportunity and cleaned out the house.
So tell her to make a plan!
As far as the house.... she has just as much rights to it as he does... possibly more if she has been paying for everything all along (I'm in MD, so I'm not familiar with how DC divorce laws work). And if he wants to come home, she probably can't stop him... technically.
But I can tell you that my X walked out on a Wednesday night, and the locks were changed on the door by 7 a.m. the next morning. Could he have gotten a locksmith and gotten back in? Sure, but I just stood my ground that he wasn't to come home. She can work with the lawyer about possibly getting exclusive use of the house until things are settled.
At one point, my X did start making some noise about trying to come back even though I said no. He ended up dropping it, but I had a plan for if he pushed it. I was going to inform him that my mother was going to come live with us indefinitely. I have no doubt that she would not have even had to show up - just knowing that he might have to live under the same roof with her (he was afraid of her... not sure why, but hey, it worked!) would have been enough to keep him away.
Sorry, that got kind of long. Bottom line - tell her to make a plan of what she will say if he wants to come home. And then stick to it! This is an emotional situation, but emotions are what lead to making really bad decisions in the heat of the moment.