This is not legal advice, just my common sense talking.
Let's face reality here. There's probably a ton of money and assets that he and his wife hold jointly that she'll get no matter what and Texas is a community property state so she already owns half of everything he's got.
In his present condition, if your husband's family convinces him to make a new will, there will be a 55 year old woman with millions of dollars who will contest the new will in a heartbeat and probably be able to throw a lot more money into it than your husband's family could.
And the family is delusional if they think there's any hope of challenging his old will that was made when he was younger and healthier.
My vote is for a quitclaim deed on the one property.
There's no reason why he can't sign a valid quitclaim deed when he's on strong drugs. His doctors and medical records can attest that he knows what he's doing. Better yet, have a lawyer that you pick out of the phone book at random handle the documents and signing and the lawyer can attest that he knows what he's doing.
It's a lot less likely that she'll challenge the deed to one property out of a 10 million dollar estate.
Another alternative is to just accept the fact that the old guy found himself some young boojum, enjoyed the heck out of her, and is leaving his estate to her. He's allowed to do that, you know. No matter whose sensibilities it offends.