You shouldn't have someone who has a romantic interest staying with you while you are in a rel@tionship.
Other people will talk about trust and you aren't interested in her and blah blah blah, but I think that if you are serious about tallgirlfriend you should have appropriate boundaries with respect to other females and that includes not having one live with you that is interested in you.
I would put a slightly different spin on it.
I would say that it's not a good idea to have someone who has an unrequited crush on you staying in your home no matter what the respective relationship
It has nothing to do with trust, or commitment to your own relationship
. Part of it is to be kind to your crushing friend. When you have a one-sided crush on someone, it's really easy to see absolutely anything as a sign of hope - "He cares about me! He really does!" so you have to be careful to avoid behaviour that would make it seem like they have a chance. Taking them in after they break up with their boyfriend would definitely fit.
The other factor is protecting yourself from unpleasant situations that result from their own feelings and self delusion. Sharing accommodation with someone who is hopelessly in love with you, and doesn't accept that you have zero interest in them, is a recipe for a really messy situation, no matter how trustworthy you are.
Say, for example, that she declares her undying love and tries some sort of ill-advised Hollywood romance/seduction gambit to lure you in, after she's left Mark. Picture yourself trying to eject a hysterical half naked woman from your apartment in the middle of the night because she won't take no for an answer.