I think if your in a serious enough relationship sometimes you have to be the one to point out that weight gain is a problem but don’t say because the look of her because truthfully that’s fucked. Instead say, your weight can affect your health and that you are worried about her and you liked that she had more confidence in herself when she was taking care of herself, that you miss having sex with her, and that the working out may be hard to start up in the beginning but it is shown to help depression because the endorphins you get and the general confidence boost. Then you need to relate to her so she doesn’t feel attacked even though she may still feel badly about herself. Tell her you know she is struggling so you will work with her to get back into a routine and be her number one support system. Don’t become pushy and don’t try and force her. Start small do 3 days a week of a workout together it can even be like 30 minutes where you cycle through a few body weight exercises. Until she starts feeling strong again and more comfortable going to a gym!