he has 4 choices
1) put up with it
2) get the friends agreement to put the house/flat up for sale in order to sell and end the mortgage. Bear in mind that if there are any penalties to pay on the mortgage they will have to take this. also, the place could be in negative equity and they may not cover the mortgage if they do sell
3) if either of them can afford (in the lenders eyes) to have the whole mortgage themselves, then one of them could exit the contract and the other one take on the mortgage.
4) if they are living together at the place (your use of the example of cleaning and bills makes me think they probably are) then one of them could get a lodger and move out, or they could rent the whole place out (bear in mind, the mortgage lender will have to agree)