If the young woman in question was of the age of consent, then the conduct, however distasteful to me personally, was probably lawful.
ON THE OTHER HAND, if she was underage and their relationship was anything other than platonic, then we are discussing criminal and predatory behavior in almost any culture or jurisdiction. If that was the situation (and Angry Father isn't clear on this), his current relationship is irrelevant to his future behavior. These guys are on the hunt and stay on the hunt until they are stopped. Many, if not most, sexual predators I've dealt with over the years were in significant relationships and still continued to seek out victims. Allowing such an individual to continue training and teaching speaks volumes about that dojo and its' leadership.
Since Angry Father hasn't been entirely clear, I won't throw stones, but this situation has alerted my Spider Sense.
one other thing might be an issue. Even if the original relationship was platonic was the girl "groomed" to enter into a relationship when it was legal? I have seen this happen both in Aikido and my professional life in higher education.
On the other hand if the girl was an "old" teenager (19 or so) althogh the situation may be distatsteful to the OP this may just be acase of mutual attraction. My daughters partner is 10 years older than her, and one of my best friends wives (who he met through Aikido) is 20 years younger than him.
All situations are different