Given that they've been married for 20 years, I'll assume that puts her in her 40s or 50s. Not uncommon for the sex drive to go through wild things at that point.
I recommend straight-up communication betweeen the two and/or counseling. There may be all kinds of issues at play here.
As far as it being a sign of her cheating, that might be, but just as easily it might be her own libido, body-image, something between the couple, et cetera. Cheating is only one of many, many reasons that partners stop being intimate with partners.
Finally, perhaps your friend can come to an accommodation that allows him to stay in the marriage but still get his needs met elsewhere? In many marriages, sexual intimacy is only one minor component, and shitcanning an otherwise good relationship over one facet is counterproductive. (Granted, my friends and acquaintances tend to be in a different category, but I've seen a fair number of relationships/marriages prosper in spite of one or both partners "stepping out" for sex.)