I would also add a follow-up comment to feedback you left for the buyer which can warn other sellers he has done this.

Don't mention anything about cases being raised as this is a policy breach and he could have the FB comment removed.

Just stick to plain facts. I think if another seller saw this they would know what it meant.

Maybe someone else might suggest better wording.

Something like:

<UID> provided an alternate mailing address via messaging (date).

Where <UID> = the user's ID so that its not hidden by ebay. Means all others can see his user name too.