It has nothing to do with you legally.
However, given his situation, he should be looking at doing a deal with the lender to sell the house and get the shortfall written off. If they know that he has moved in with someone who has just bought a house for cash, it will smell wrong.
There is nothing wrong with it, but I would hold off until he is sorted out.
There is a small possibility that the lender might write off his shortfall if he (i.e. you) paid €10k towards the shortfall. Would that be worth avoiding bankruptcy for?
Brendan