I adopted 2 four year old girls almost a year ago who were not house trained. I trained them exactly as I would have with a puppy. Take them out frequently, nice treats when they perform and watch them like a hawk. I still do not trust them to be in another room on their own and perhaps I never will be able to trust them. They are 99% reliable now but the odd accident happens if I am not vigilant. I am training them to use poochie bells - Bella will ring on command and even rang the bell herself the other night to go out and do a poop
Best of luck if you go ahead with the adoption.
Molly (Blenheim) & Bella (Tri)
Honey, Cassie & Tiki (Cavalier Angels)