options:
1) let it out for enough to cover the mortgage. This means she can keep the house until perhaps being able to afford it herself. The problem is that if you do it by the book the lender will not play along. If you don't tell them it probably won't matter as long as the mortgage is being paid. Divert the mail in the mean time.
2) Sell the house but that will involve a myriad of fees.
3) Live in it and let the rooms, ok if she can stand having strangers in the house.
The boyfriend may also have a claim even though he isnot actually on the docs.
I'd probably do option 1. no doubt someone will point out that it's fraud yada yada yada. It's very difficult to do things like this by the book, lenders will want too many hoops cleared.