Apple Watch question

Please help me girls!

My bfs ex won’t leave my bf alone and I have one of his Apple watches he gave me except it’s connected to his cellphone and we live a few hours away. He doesn’t know this. The thing is.... I still can receive the calls but for some reason I don’t get msgs unless I’m close to his phone. Is there any way I could fix this so I can see if she’s texting him. I noticed he doesn’t call her back even when I’m the one ignoring her calls. But I’d want to see if he’s getting any text msgs and if he’s replying to them.