I'm so sorry you are having to go through this.
But get the loser guy out of your home. Only you allow yourself to be taken advantage of. It's still your home, your rules.
The letter idea is good and maybe talking to her about some counseling. Maybe through counseling she can find out why she is drawn to losers and learn to make better choices.
In the meantime, it may be time to get tough. My dad went through YEARS of this with my brother and my brother never changed, until my dad said he was done and washed his hands of it. Then he did change for a few years, but lately has gone back to the way he used to be. But my dad didn't cut my brother loose until he was almost 50 years old. So the only advice I can give you is don't let it go on so long that the possibility of change ceases to exist.
Sending you hugs and hopefully a good nights sleep. And if your daughter won't go for counseling, then you go. I am a firm believer in counseling (if you have the right counselor and I went rhough a couple before I found the one that I had the best fit with).