Returning a dog?

So my MIL and FIL chose to get a puppy from my dog that had them about 10 months ago. I told him in the very beginning he could keep her for a day and if she was to much he could bring her back the next day. Well time passes by and they love her. One day he let her out her kennel and didn't take her outside went in his room shut the door and went to sleep. She chewed a hole in a rug then told my MIL they need to return her immediately. How the hell do I explain they can't just do that besides just telling him no. I personally feel like it's not her fault he felt her out right after getting home without doing anything with her. She's very very little and has a ton of energy. The hole she also chewed is the size of a quarter.