Not really attracted to spouse anymore because of weight

My cousin is having marriage issues. His wife has been getting more and more overweight. She just doesn't care. He doesn't find her as attractive anymore. He says he loves her some days and some days he just wants to be alone. It's so hard hearing this. But that's just how he's feeling. And I'm glad he's able to open up to me about it. We are best friends. He says they're going to go to a marriage counselor. But how should he approach this? How should he tell her how he feels? I don't know what to tell him. 😕 Please be nice as he means a lot to me. If you have questions, I'll be posting here since it's anonymous. Thanks