My wife(38} has always been thin, and eat very little she has gained weight & can't lose it, she eats even less now still no change?

Smarter eating: At 38 yo it is possible that the hormonal changes are contributing to the weigh gain. Unfortunately when a person starts to eat much less the body feels like it is in starvation mode and it holds on to the calories and makes it much harder to lose weight. She would be much better off eating 4-5 smaller meals each day instead of a few small ones. Add consistent exercise and it will make a big diffe.