If an athletes girl suffer from amenoherra should i make her stop exercice at all or reduce it?

Not your option: I see that you are a 19 year old man. Being aerobically fit / exercising is a good thing and does not cause amenorrhea, though lack of body fat may contribute. There are a variety of other reasons a woman may not get her period and it's not for you to diagnose. This is a problem for her physician to manage with her, and it's not something over which you should try to exert any control.