I'm feeling stuck.....

So I recently discovered my ex is in a new serious relationship. He refused to tell me he was seeing someone. (We were only separated 2 months and he was still having sex and leading me on to believe we had something there) I'm convinced he was seeing her before we separated. I'm having a hard time letting go n moving on. He wants to be friends and act like nothing. Still wants to deny his new relationship. A friend showed me her fb and they look serious. But he still has me in a relationship on his...... He has blown off his kids n has been caught up in so many lies ....... I want to reach out to her to confirm how long it has been going on n to point out his ways. Is that wrong? Would you reach out?