My fiance (24f) cannot gain weight. Even tho shes trying, im worried about her health

We have been together going on 6 years and she's always been pretty small. She put on some weight when she was pregnant with our son and was. A healthy 130is. But after giving birth her body snapped right back to about 100lbd and almost 4 years later she's struggling to get back to even 110. And she eats pretty good but skips meals at work. How do I help her