I would advise my friend to first address the lack of intimacy, not assume he is cheating. There is a real problem if there is no sex in a marriage (unless there are health reasons). They need to talk it out and make some decisions to spice up their marriage. If there are deeper issues, they should get professional counseling.

It's not your place to say if she should leave him or accuse him of cheating (even though he may be cheating), but I would encourage her to try to first work things out with the help of professionals (perhaps counseling or maybe even a Viagra Rx from a doctor if there is lack of sex drive).