If I were you, I will not be happy with these emails.

I will avoid platdate with them later, and stay away from them. There are so many other kids around, your daughter can find other friend.
I will let my hu*****and know I am not happy with this, and let him reply the email like "Ah, my wife is interested how you made those plank, and want to learn from you" blah blah, to let her know that you and your hu*****and has no secrect to each other.

I only make friends with those whom I trust.