First off, ANY time you have an opportunity to purchase a property "subject to the existing financing" (financing must be good i.e. not adjustable rate) take it! This is all but giving a house away.
Like @Josh James said, get this thing locked up into a contract to ensure that it's your deal. In the contract, make sure you include that the purchase is subject to the existing financing. Also make sure you give enough time for the title company to get the mess worked out when coming up with a deadline. If you need to, you can make the appropriate addendum's to prolong the contract should it take even longer to get things worked out.
Don't rely on the seller to do YOUR due diligence. After you have it under contract, contact a trusted title company ( I would contact the title company he is supposedly working with first), give them everything you know on the situation and let them advise you on what to do next.
P.S.
**Include "seller must convey clear and marketable title to buyer" in your contract.**