is he too young for MLC?

[Dom R]

I'd say, that a man and a woman going to a concert, or dinner out, alone together, is a date.

So, as far as your friend goes, you might mention if that is your opinion to her... get her feedback on that... and then take it from there.

how I would handle it if it were me:

Ideally, she would agree. But even if she doesnt... as long as she agrees to respect YOUR view in that manner, and agree that she is done "dating" your husband, I would be willing to forgive her.