Hard and fast reality check, no beating arond the bush. What is with the comprise mom? Any good parent would not question what to do. You sound like a good parent. How can a parent even make a choice between their child's welfare and a recreational activity. A 40 YEAR OLD MAN has no business with teenagers. This is an emotional and personal issue for me too. As a father who does aikido I would not hesitate to leave that dojo instantly.
While I agree that grooming or behaviour of a sexual nature is clearly out of order if the girl in question is underage , if the girl in question is over the age of consent and both parties are happy to be together in a relationship built on a sound foundation, pray , make I ask , what has age got to do with it?People are human, what may well attract one person may well repel someone else. Older women , some quite famous, now marry much younger men.My own view is this, if two people care and respect each other and are of legal age what business is it of anyone else? A father or mother may well have reservations but in the end its the man/ woman who makes the decision.If the relationship is sound it will survive if not both parties will split up.This of course is quite normal in couples where the age difference is not so great.