Okay, we have a couple things going on here.
FIRST the O/P is NOT on their initial entry of their ED visa (seeing as they entered the country back in Dec 2017. This means they are on an extension of stay from the immigration office.
When they left the country they were stamped out BUT they didn't cancel their extension of stay.
When they come back to thailand they will get stamped in on a 30 day visa exempt entry stamp BUT as
@Glen ********
correctly mentioned, they still need to go to the immigration office and cancel their ED visa extension the "official" way. Failing to do that will cause them a problem and could subject them to a fine too.