It's normal for men to wank, but it's definitely abnormal if he prefers to wank over the real thing.
It always takes 2 hands to clap; there surely must be a reason why he prefers not to have sex with his wife.
I'm not a qualified counsellor, but i suggest your friend to have a heart to heart talk with her man.
Or try something new to entice her man's interest in her once again.