How do you justify cheating?

I have just been reading through some answer’s to another question on here today. Everyone seems to have strong opinions about cheating, but I would like to put another side to this.

I have a friend he is South African, a moral person, a religious person and a very stable and loving individual. He has been married to his wife for over 27yrs, they moved to the UK some 14 yrs ago. Within 6 months of being here, his wife was involved in a very serious car accident. Luckily she survived, but suffered some serious brain damage, which now leaves her in a condition where she can no longer look after herself competently at all, she has the mental age of a 6yr old child and does not function like a fully grown woman anymore. Her husband no longer has the marriage he once had to his wife. He stays and looks after her, he hires caretakers on a daily basis and gets on with life day to day. Obviously there is no marital bed to be shared and they can’t be a couple anymore because of her condition.

Some 9yrs ago Chris (the husband) met a wonderful woman and fell in love all over again, before he started any relations with her, he explained about his wife and said that he would never abandon or divorce her and if they wanted anything to happen then it would simple have to be as it was.
He told his wife’s family and asked their permission if they would give their blessing for this relationship to take place on the assurance he would stay married and look after her. He got it.

Today he lives with his lover and they have 2 small children together.

So people tell me is this classed as cheating? Is it wrong? If not why not? If it is then why is it?