Sacked For Sick Leave While On Holiday?

[EDDIE51]

My son works for a large multi national company. 5 weeks ago he dislocated his shoulder trying to move a large very heavy piece of the firms equipment.
He has not been able to work since, he has had doctors and hospital certificates for every day of his sick leave and has gone out of his way to inform the firm of the situation as he has each doctors appointment and test.
Today he was told by the hospital that he has nerve damage and a possible hairline fracture but more tests are needed.He has been signed off for another 4 weeks. There is a complication, he is due to go on holiday to the USA on Tuesday the 6th. ( booked over a year ago) The company say he should cancel his holiday in case they can find him 'light work' that he could do while still injured.
The travel insurance co doctor says that the injury is not sufficient to justify canceling his holiday and he is fit to travel but not to work.
The company are now saying they would have grounds to dismiss him if he goes on holiday while on sick leave. They have accepted that the injury was caused by work and that there was not an instruction in place as to how the equipment should be moved.As it is a 'work related' injury they have to pay him until he can return to work.
Can they do this (dismiss him) when they have accepted that the injury was at least in part due to the company's lack of instruction as to how to move the equipment? Can he go on holiday while off sick?
He can't go back to work voluntarily as his doctor still says he is not fit to work, He was intending to go back to work and take the 2 weeks as holiday to save the problem. Sorry for such a long post.

[EDDIE51]

At first the doctor told him he could work as long as it was on 'light duties' . When he reported for work he was told there had been a breakdown in communication and there was no 'light work' available. He was told he had to do his normal job, he agreed to this but it made his injury worse.
With hindsight of course he should have refused to do 'normal work' and gone home again. However he did do it under protest and it made his injury worse.
It was shift work at night and they told him he had to work as they would not be able to get a replacement at short notice and they were already short staffed.
What annoys me is that he has never 'taken the p*** over sick pay and goes out of his way to be as helpful as possible and make certain the company are kept fully aware of the situation. If he had been less honest he could just have put in the new 4 week sick note and gone on holiday with no one any the wiser. He is taking his mobile phone so the company would be able to contact him. ( not sure if they would know if they called him that he was in the USA?)