His Friend Cheated On His Girlfriend With Multiple Women. Should He Tell Her?

Some people just don’t get it:

My childhood friend cheated on his gf last summer with multiple partners. He just told me they’re about to get married and she still has no idea.
Infidelity

My childhood friend Brandon and I grew up together and our families know each other. He’s like a big brother to me. He’s been dating his current girlfriend for 5 years, and he was absolutely obsessed with her in the beginning of the relationship.

What happened: