Hi Cathy. This can be a touchy subject with some banks.
Most lenders who approve a short-sale will require it to be an 'Arm's Length Transaction', meaning that no 'blood' relative of the seller can purchase the home. Both buyer and seller (and their respective real estate agents) are typically asked to sign an affidavit stating that all parties agree that the sale is an 'Arm's Length Transaction'. There are penalties to all parties involved should the bank determine that the affidavit has been violated or entered into fraudulently.
Since you aren't married, and never were, and you're not a 'blood' relative, you should be able to make an offer and purchase the home IF your offer is sufficient to meet the bank's minimum NET. However, since you were on title at some point, albeit with different last names (I'm assuming), even though you were never on the loan, they might NOT consider that to be 'Arm's Length' should they look at the title history, which they might.
That being said, before you wasted your time and energy making an offer, I would have the seller call his lender and simply ask a few questions about what 'type' of buyer is allowed. Be up front with them, and try and get their definition of what they consider to be an 'Arms Length Transaction'. To be perfectly honest, the seller's personal life is of no consequence to the lender, so long as it doesn't violate the original loan agreement and deed of trust, at least that's how I view it.
I'm sure there will be varying opinions from the agents here on Trulia, however, but that's what this forum/blog is for!
Hope that helps!
Century 21 All Star, REALTORS