As pointed out in comments, one can always write [email protected]. From my own point of view, that would be the preferred option.
If it's something quick, a user can raise a second flag. I've seen this done when a user felt that a prior message in a flag hadn't gotten through or was misunderstood.
A third option is a private chat room, as mentioned in a comment. But that's something initiated only on the moderator end. See https://meta.stackexchange.com/a/143645/278664