You probably cannot; but it doesn't matter:
You cannot just transfer the Title to your name, you would have to take the LOAN too.
And you couldn't take the Loan, because he is Upside/Down, and the Appraisal would prevent that from happening.
You would have to qualify for a NEW loan, and someone would have to pay the Bank for the difference.
Homeowners have been trying to short-cut this situation for 3-4 years now:
Beware if someone tells you that they know how.
Your friend has basically 4 choices:
Keep the house, (somehow) and wait till it comes back up,
Shortsale the house
Let it Foreclose,
Do a Deed-in-Lieu
Good luck and may God bless to you both