If the doctor has signed you off you should be off work.
For the first five days of a sickness you need to fill in a self certification of sickness, after five days you need a certificate from your GP.
If the illness is caused through injury at work he needs to contact the HSE (Health and Safety Executive) and report the happenings. I am sure the company think they are using the correct procedures, but it looks likje there is a case for constructive dismissal which would entitle him to reinstatement or compensation.
Don't quote me though, this is only as far as I can work out and I'm sure it is accurate. But to answer your question directly... Yes, I think he has a case for unfair dimisal.
As Whickerman said, Post again in Law and see if there are any clearer answers.
Hth
Pete