Truthfully he's lucky she didn't chop to her ears or get a pixie. Waist length hair is still quite long, and a lot of guys would be thrilled if their wives/girlfriends wore their hair waist length. Many many men have had to deal with their wives chopping off their crowning glory for an easier, more practical, more mature/sophisticated, etc., look. It's hard, but most guys get over it. Once the hair is gone, it usually is never grown back.
Your friend should talk to his wife. If she doesn't know how much he loves her hair, it's his fault. If he doesn't at least offer to help her take care of it, and compliment it frequently, it's his fault. There's a chance she will accommodate his wishes if she knows them. There's also a chance that she's planning on cutting her hair in stages and that this is only the first of several cuts that will eventually lead to short hair.
Unfortunately, once a women gets it in her head that it's time to chop the long hair, it is nearly impossible to convince her otherwise.
A number of people believe that when a woman chops her hair, she's showing less interest in sex, or at least attracting men. For example, recent brides frequently chop their hair because they don't have to worry about getting attention from guys any more.
Some good articles on the topic are:
dailymail.co.uk/femail/article-1092019/Does-short-haircut-mean-women-gone-sex.html
http://www.medindia.net/news/Does-Cutting-Your-Hair-Make-You-Lose-Interest-in-Sex--44892-1.htm
There are some advantages for a husband of his wife cutting her hair short. She'll get less attention from other guys and take less time to get ready. Very long hair can also get in the way during sex, which is annoying to some people.
Your friend should try to support his wife and try to have sex with her. If her very long hair was his primary turnon, he may need to find other focal points, such as her breasts. But with waist length hair, her hair really should still be long enough.
He, and other guys blessed to be with very-long-haired women, should also "practice" getting turned on without the hair. Nearly all women with long hair eventually cut it, so it's best to be prepared. All this takes is having sex once in a while with her hair up in a bun or tied back in a braid (and also not focusing on the bun or braid during).
This cut may have just been a periodic (large) trim. But if she is set on cutting her hair short, he might be able to get involved and ease the shock that way. Her telling him in advance when and how much she plans to cut her hair can give him some time to get used to the idea and "say goodbye." He might be able to help her pick a short style that he doesn't dislike as much. Another thing that may seem horrible but can actually help is to get involved in the cutting. Think of it this way: if someone is going to cut 12, 18, or 24 inches off of his wife's hair, wouldn't he rather be the one to do it than some stranger? If she decides to go short, he can cut the braid at home and then go with her to the salon to finish uip, so the hard part can be done in private. Him keeping her cut braid as a memento may also make it a bit easier for him.