I went through that and I worry for your friend. The problem with this kind of situation is that you feel you live something special and also above anybody else's way of thinking "they cannot understand", "this happiness is so unique"...
My ex-girlfriend didn't cheat on me but started to be friend with an idiot hitting on her. She as a woman always said that she loves sensitive men and hated players which makes it more difficult for me to accept. This woman refused her femininity as much as I refused my masculinity. I saw myself so much hurt by her disrespectful and humiliating behavior that I decided to be disconnected emotionally towards her which led to a lack of feeling and lack of desire, I started desiring other women so decided to break up. To my surprise, she was hurt by that and blamed me for not loving her anymore. I only told her the real reasons of my feeling change a year later and seemed to deeply regret it especially when this guy made a business deal with her and ruined her. What was interesting is that she recognizes that her behavior was wrong but couldn't explain why she acted like that with me. (my interpretation today after reading women psychology, is that her ideological an intellectual models fight the female animal in her, I represented the intellectual call (sexually though, I was a good lover

) but the animal in her was looking for that alpha male. Because of this artificial conflict she went to the wrong alpha that caused her so much trouble)
After my break up, I wanted to enjoy light relationships, but being a beta-AFC and not knowing how to approach women makes it difficult and creates frustration. I managed to have only one girl that I slept with only one time and gave me herpes (despite the full protection during sex).
I'm 35 now, went through a depression and seeing a psychologist trying to figure out what's wrong with me.
In conclusion, your friend needs to be careful and in touch with his needs. Even if she is not the kind of girl that cheats (like my girlfriend was) she still can act in a humiliating or hurtful way, and being a beta will force him to accept the unacceptable from her
good luck