Any thoughts on how to get to change a dog's thinking about bathroom spots?

BF and I have been together 14+ months, we both have dogs. BF's dog Sara is 13. Sara nearly died in January from liver toxicity from Rimadyl. Happily, she's recovered now, but a problem has developed. When we first had her home, we were giving her fluids subcutaneously. One night we loaded her up with fluid, and she had to pee in the middle of the night. There's a partially finished basement downstairs and we usually let the dogs out the back through a slider. She went downstairs; when there was no one to let her out she went to the far end of the room and peed (reasonable given that she'd just been loaded up with fluid). The problem is that ever since she's considered that spot of carpet an alternative bathroom spot and both pees and poops there. She doesn't do it anywhere else in the house and doesn't do it at BF's house. I've tried shampooing the rug to reduce the smell, but it didn't seem to help. Any thoughts? (BF is planning to move in later this month so we're worried it will happen more frequently.)