Is it still a marriage? Sounds more like they are cohabiting. Legally I guess it is.
The weight isn't the issue although it doesn't help. The issue is boredom. They have ground the passion out of each other or life has. I am sure she still wants sex, just not with him. He still wants sex. Neither of them are working very hard at making the other feel wanted or sexy. Why is the question being asked now 3 YEARS after the sex dried up and not 3 days or 3 months.
Normally I would say apply covert dread but that is pointless if the other person doesnt give a f-ck.
Some people have little or no sex drive. Even so, they both need to get passionate about something.