I read about this yesterday, the ironic part is, he could have been disciplined for going on holiday whilst on sick leave, however, they may have shot themselves in the foot by stating it was about his physical condition when he was suffering from a mental problem.
Don't get me wrong, I think the company are assholes for sacking him, but what they did wrong here was largely procedural (and possibly level-of-response) error under current UK employment law. He should have been given a written warning at worst.
The real ironic part is that the charity is for Children. I'd be very worried about sending my child to a 'Boys and Girls Club' where the staff are sacked for saving childrens' lives whilst being ill...