Putting things around the area won't work if this dog has a stubborn temperament. My german shepherd practically laughed at us when we used ammonia, vinegar or lemon juice for training when he was a pup. Seriously, we were trying to deter him from excessive barking and the dog trainers told us to squirt him in the face. Yeah, his whole head was soaked and he still just kept right on barking. He is terrible about marking his territory outside and trampling everything, so I wanted him away from the pond too. Those little metal garden gates didn't work at first, then hubby attached them to wood stakes to make them stronger and that kept him out till we got the underground fence. Now the fence works, but only as long as he isn't really set on going past the line. If my husband is on the other side, or his ball, he's going over there, and no shock in the world is going to stop him. But for the most part it is working. I bought mine online from DHGate.com it was only $80 for the complete system with 2 collars for 2 dogs. Its like an Ebay for wholesale stuff and there were several on there with just one collar that were in the $50 range. You just have to go through the listings till you find one with no minimum order. I would not invest in a $200 system for a dog that visits twice a year. I would then make it clear under no uncertain terms that if she brings her dog to visit the collar goes on and stays on or the dog goes to a kennel.
My sister moved in with us last spring for a few months and brought her dog. Same thing, she thought her dog was a saint despite the fact that it was using my house as it's personal bathroom, digging holes all over my yard, chewing up the kids' toys, and then the last straw was when it tore up my custom fit patio cushions. She thought I was a monster when I told her the dog couldn't live here anymore. And then this year when I had to fork over $200 in new cushions, do you think she pitched in? Nope. She even had the audacity to try to say that I had no proof it was her dog over my own! My dogs who are 7 and 5, have been to obedience school, have never chewed up a single thing or ever relieved themselves in my house (with the exception of Sebastian when we learned he has a sensitive stomach and couldn't eat regular dog food). And she still throws it in my face from time to time that I made her get rid of her beloved dog.
Part of being a responsible pet owner is making sure your pets don't damage other people's property. I wouldn't hesitate one bit to lay down the rules when it comes to your FIL's girlfriend!! And your FIL should have enough respect for you and your husband to back you up as well.
