Fiance dismissed for gross misconduct

I'm requesting advice for my partner. He just got dismissed from his much loved job and is too upset to deal with anything. Sorry if this is long and complicated.
He worked at a dog day care centre as a driver and last month was suspended after he made a couple of silly errors. They are saying it was gross misconduct but I think they are being harsh. He was under a lot of pressure and stress at the time which led to a lapse of thinking. He left some dogs unattended for 3 minutes but they were in sight of other staff members. His boss is asking why he didn't call the site manager to report it, he said he used his initiative and experience to judge that they would be ok and that he had told his colleagues to go attend to them. The other mistake he made was he forgot to take 2 dogs out of his van to drop off at the centre, but he realised quickly when he was on his next collection and kept checking on them to see if they were ok. Again his boss wanted to know why he didn't call them. I don't see how calling his boss would have changed the fact that the dogs were in the van for longer than usual, which is their issue. My fiance argued that sometimes he can be stuck in traffic for over 3 hours and not be able to check on the dogs' welfare but then they contradicted themselves by saying sometimes things are out of our control? So it seems to be fine sometimes for this to happen if it suits them!
I am so angry with their behaviour. They didn't give him a verbal warning plus his English isn't great as he is from another country and I'm worried he misunderstood things that was discussed in his disciplinary. He has been a very loyal employee having worked there over 5 years without a day off sick. The last 5 weeks he was made to train a new driver everyday and most of the time never had a lunch break so he was exhausted. That day he was also stressed because he recently had an accident at work where he chased after a dog but broke his knee and was due to have leave for surgery, which they gave him a month off for but he wasn't able to walk properly and was in pain and had been waiting months for the surgery due to his employer not allowing him the time off for surgery straight away. They decided to suspend him the day before he was due to leave for surgery so when he returned they told him he was dismissed. Also, his own dog had recently broken it's leg and was in the process of treatment which was stressing him out and also his father recently passed away. I'm wondering if he has a case because as an employer they did nothing to control stress levels which I believe played a part in his lapse of thinking at work. (Maybe also the injury from his job is something else to pursue??)
Also, he told them that he witnessed other colleagues making the same mistake as him by leaving dogs unattended but nothing was ever mentioned to them. I'm wondering if him having so much time off for his knee operation might be why they want to get rid of him?
I'm glad he's not working there now as they have been treating the dogs and staff badly and just seem interested in growing the company and making money. There is a high staff turnover too.
What do you suggest? I know his clients will be really upset and angry, most of them have said they would leave if he left as they value him so highly. His boss has offered him excellent references and 6 weeks paid leave.