How do you tell a friend who hasn't had sexual relations with her husband in years that her husband is cheating?

Sounds like your friend has a pretty bad relationship with her husband and may be staying with him for family or lifestyle reasons. Maybe she needs to know the truth to help her confront her own life and her own happiness. What about a letter, anonymous. This way she won't feel awkward with you.

What makes you think she doesn't already know? If she's not having sex with him, she should suspect that he is getting it somewhere.