Why on earth would you do that. Your friend may be the most honest guy in the whole world but why do you want to test that theory on your own dime. My unsolicited advice...

In any case, to answer your question, the person assuming the mortgage (you) should be able to a) pay 20% down; b) prove that the monthly payment would be less that 31% of the take home salary; c) should have good credit history; d) should be able to provide last 2 paystub.