1) It is porbably for your friends best interest to get out of the house anyways. THink that he did her a favour by kicking her out, and start over new.
2) if the girl decided t leave. let her go. When she comes back, welcome her back without any questions or judgement.
3) Your parents need to take care of themselves. it is not your problem to solve.
I think you should work on building your life rather than patch your parents life up.
No parent would be happy to see that a child sacrificed his/ her life for the parents.