Should a 51 Year Old Male Learn Muay Thai?

Asking for a friend who is in Chiang Mai. He is 51 and has fully recovered from a knee surgery. Being sedentary all this while made him gain weight and he wants to learn Muay Thai to get back to the level of his fitness.

He is reluctant to join a gym because his knee injury occurred during a lifting snatch session.

Is it okay for him to get into Muay Thai training per his age and physical condition. Doc said not to carry heavy weights at least for the next 7 months. Is his age and condition

Thanks.