It’s not helping at all, it’s moving the problem to be someone else’s and abdicating XC’s responsibility to its passengers.
It’s poor practice at best, and I’m sure contractually ‘iffy’ too.
But, realistically, what's the solution?
The TM can't pull another 4 coach voyager out of their cash float to tag on the back, so extra capacity is out of the question.
Refunds wouldn't be offered by XC and no one at XC management will give a hoot about it (remember, 2010-2020 XC was 'get on the train if you're lucky' levels of capacity).
So those options out of the window, I don't think there's any harm done by offering anyone who'd care for some more space the option to disembark and take another service. Admittedly, yes, the TM should have specified that you must not be on a train specific ticket, but otherwise, I don't see the problem with a colleague thinking on their feet and doing what they can to help people have a more enjoyable journey (which, in this case, was to offer an alternative option).