He's got two and a half months left of his contract with BT and would like to go back to ADSL2+ but seems he might not be able to.
I would say it can be done. But there is a higher risk that it won't be hassle free and that some chasing might be required before it all gets back to being operational. There is also a good chance that all will be fine. Luck of the draw.
If he doesn't want hassle then stay with fibre. If he has a particular driver (ie cost) and is willing to risk having to do a bit of legwork if something goes wrong then go for the downgrade.