How do woman tell other woman how they feel?
Women find it easy to talk to other women they trust.
Dump him but tell him you just don't feel comfortable being the woman he's cheating with. Remember if the other person finds out you could be blamed even if you didn't know he was a cheater. Also if you really like him tell him you would feel better dating him when he isn't dating someone besides you
He's disrespecting you & your relationship. He can't particularly say she's just a friend because he felt the need to hide her from you. Tell him how you feel & express that you don't feel comfortable with him writing other women, especially those he doesn't tell you about. Be direct & up front, otherwise it'll continue to happen.