Friend not having period but TTC, help her!

So my best friend is trying to get pregnant but she hasn’t had a period in a few months. All her tests have been negative so it’s really disheartening for her because she wants to be able to track her cycles so she and her husband can try for their second child. Their first child was unplanned 7 years ago and she has not been pregnant again since then. Her OB says it’s because she needs to gain weight but she has been eating and trying so hard and can’t gain the amount of weight he wants her to gain before he will help her with the fertility issue. She is only underweight by about 5-10 pounds and it’s due to her genetics, she has a very fast metabolism.

Does anyone have any tips for her? I never had fertility issues so the only advice about getting pregnant that I know is useless to her right now. I just want to help my friend, her heart is broken because she and her husband are trying so hard with no results. I feel so bad because I didn’t know they were trying and I ended up pregnant with my third child and when I told them they were happy for me and opened up to me about their fertility struggles and I wish I could help in some way.

For more context, they’re in their mid-20s, married, employed, and both in great health. Their first child was unplanned so no information about that cycle. Her last period was about 3 months ago. All pregnancy tests negative. Any advice to get her cycle to restart?