That's a huge over sight.
It is uncommon for plumbers not to glue this piece so there are some options.
When you say your tile guy poured the concrete - this was done over a rubber liner? What type of drain is it?
Did you do a flood test first? 1-3 days. if the answers to these questions are no-no-no.
I think you are hoping someone tells you it's OK. I think you know the proper answer and that is to rip it out and make it right.
If the tile guy has only done the pre slope then it's no big deal to chip a little mortar and fix the drain. If the shower has a rubber liner then you could cut through the mortar around the drain - fix the drain. And patch the repair using a Noble Flex Flashing. Then you would continue with NobleSeal TS or DalSeal TS and go a topical membrane route.
I'm thinking do over. JW