I think an email is fine but it would have been better if you had done it Friday night with the idea of saying how he walked out before you could finish talking and so you are sending the rest of the conversation by email.

At the same time you can still do it by email using the same reasoning that pickups are too quick and not a good time for having a "complete" conversation so you are doing it by email. If you have the email for both parents then send copies to both parents. If dad is always doing the pickups maybe mom is only partly aware of how deep your concerns are.