Hi @Jeffrey Mark,
I have had this situation arise... and its annoying, as a wholesaler. What I have found, too, is that you may be getting tested as well to see if you are legit, or just gaming them.
I would get the offer to them and see if you can meet the seller and "partner" in person to sign the deal. The fact of the matter is, if this guy is going to come between the seller and you, there is not much you will be able to do, unless the seller asks them to leave. If the deal actually does move to close, the seller would have to produce identification to the escrow closer or notary to verify they are the legal owner of the property (or they should). If they are not actually a part of the transaction, they will be asked to either step out of the way or to verify that they are needed there at the signing.
I have also had to work with some folks that were elderly, and one of their kids had power of attorney, and not only did that person weigh in on the sale, but all the siblings had to have their say on the deal. That was just part of gauntlet I had to go through to get the deal.
I would jump through the hoop, and see how far the deal goes. The truth usually makes its way out at some point in that process.
I hope that was helpful. Cheers, and happy hunting!
Brian Truman