First train from Exeter is 06:25 and arrives at 11:00 (if you're lucky). A train back at 16:56 arrives at 20:59. So, the worker is travelling for eight and a half hours (not including his getting to/from St Davids, and any travelling he has to do when arriving in Manchester). Working for probably no more than four and a half at best and is out of his house for the thick end of sixteen hours. Not something I'd like to do more than once.
It's certainly not something that I'd want to do much either, but then why should we need to?
If there's a lot of reason to do so then it should be possible to manage your requirements to be there so that you maximise your time there whilst minimising travel. Given that even with the high emissions from a hotel (which will have come down in the last 4 years and will continue to do so over time) added to train travel making it equal to air travel then it's no worse to do that. However, that assumes a fairly low loading to the trains (30%) and even allowing for averages, first class and empty stock movements is likely that XC have a higher seat occupancy than that.
However by going by train and staying in a hotel, as there is no flight option, then it would likely mean that others wouldn't make that trip. That would mean that they have a lower footprint than they would have had.
There'll be others who would be staying over anyway and so cutting their emissions from flying is going to be an improvement regardless.
On a single example going by train may not be as green, overall it certainly is. It will also get much greener as time goes by as more of the network is wired up and more bimodal trains are used.
With HS2 and possibly even a bridge to Ireland that could significantly reduce our need to fly. As an example HS2 between OOC and Manchester then GWR for the rest of the way would be cut probably to something like 3:30. It would also likely slash the carbon emissions, meaning that even with a hotel stay it would still win out.