I understand where you are coming from. The mom going to work out at the gym during the very limited time her daughter is there sends a message that she's not important enough to spend time with. It'd be like DH deciding the only time he could work out at the gym was the 2 hours when the kids are doing homework and getting ready for bed. Of course it's not 'child endangerment' as some sarcastically implied, it's inconsiderate!
My sister went through a very similar situation with her new husband and his son. Everytime they said 'no' to him or expected something from him, he'd just go to his mom's and get his way. He even had her bring him drive-through food at his dad's house a few times when he 'didn't like' what they were serving for dinner. Can you believe that?!?!? Oh, and then when he stopped going to school she decided she's pay him $5/hour to attend. Well, duh, by Tuesday or Wednesday of each week he'd have enough money to do/buy whatever he wanted to do and wouldn't go. Even the time he was there was wasted because he didn't actually do any work.
Anyway, they ended up employed a child therapist who did a very hand's on re-parenting program with them. Sounds similar to the Love and Logic people are talking about. What it boiled down to was "Pay to Play." IF he wanted to reside in their house, eat their food, and use their resouces such as cell phone plans, car insurance, laundry, etc, there were certain things expected of him. (chores and behaviors and school attendence) Otherwise he was free to go to mom's house. Except everyone knew mom wouldn't have him around all the time so the kid had little choice. There was no budging with the new rules. And within a month, they had a whole new kid. He attended school, ate their dinners, helped out as required, etc. It was a tough month, but eventually the kid came to realize there was no way out and once he got used to structure he came to appreciate it.