Englishman,
Everyone grieves differently. Like you say, your friend probably started to grieve 2 years ago when his wife stopped being the woman he married. He obviously has been through a bad time and this other lady is helping him.
I think it also depends on who it is you lose. A loss of a parent or grandparent or sibling is terrible, but I don't think anything can compare to losing a child. I know a man who breaks his heart crying over the loss of his two young daughters over 20 years ago. He still weeps for them to this day. I cry daily for my young daughter who was killed over 12 years ago. Some people seem to get over these things more quickly, but I sometimes think they are just better at hiding their feelings. I know I'll never stop greiving for Claire - she is always with me and not a day goes by when I don't think of her and long to hold her.
I hope your friend is coping with his loss. It must be a hard time for the whole family.
Just my thoughts.
xxR