Esa mess up

A friend of mine put in a claim for esa in November but despite sending in sick notes when requested to do so they haven't been paid a penny. They eventually rang and asked why last week to be told they were a student. They are not and haven't been since 2014. They were told to send in evidence of this, which they are waiting on a letter from their last college. In the mean time they had their medical and failed.
Can someone please confirm if the money they should have been paid up until the medical is still owed to them? They're under the impression it is not owed, but surely had esa actually been paying them as they should have been they wouldn't then take all that money back.
We have no idea why they think they are a student.
Also is there a universal credit forum please? As if they move back to their parents they need to claim that and we're told last time they were entitled to zero benefits. Seems odd to me unless of course they thought they were a student then too.
Thanks

Thank you for replying. They have been told by someone at esa that they will not be paying any money owed until their appeal is heard and they win it. I'm not convinced that is correct as had they been getting the money esa would not be recovering it,and it amounts to the same thing. Any advice on what to do now please? Thanks