Well, primarily I have to ask if his attention was inappropriate ON the mat. Off the mat is no one's business but their own. If they intended to pursue a romantic relationship, I feel that speaking to the Dojo Cho would have been the respectful thing to do since he was teaching a class. It sounds like they have a steady relationship that has been ongoing for awhile and this wasn't some sort of fling designed to make him feel better about himself at the expense of the girl's self esteem. Plus, as a woman, I generally only date men 8-12 years older than me as a rule. I've found that the way my life has gone, I work better in a relationship with a man with more life experience.
On a side note, you do realize you will despise all of your daughter's boyfriends, right? If it isn't an age issue, there will be multiple other reasons for you to decide he's not good enough for your little girl. Keeping her out of Aikido for fear she will meet an older man may seem rational to you right now, but I assure you the teenage boys she goes to school with who will inevitably ask her out will be just as distasteful to you.
Thank you, Ashley, I love your answer. Now, I would like to add something:
Angry Father, let's talk about your daughter.
She's a teenager, and it might be time to talk to her about boys and men. You cannot raise her in a fortress, and if you want her to be a responsible and independent adult one day, you need to start teaching her now.
Wherever you will take her, or let her go, she will be in danger of attracting some pedophile's attention. You will not be able to follow her step by step wherever she will go, and she needs to be informed about the dangers out there. Not to mention the fact that preventing her to have some activities outside of her school is not recommended and will backfire one day.
So look for another dojo, if you feel that it will make you more comfortable, but we have a saying here that says: you know what you are leaving behind, but you do not know what you will find ahead.
You want to keep your little girl safe, your wife and you need to talk to her.