baby momma

So I've been with my best friend for 6 months now and his sons mom is "happily" engaged but she's been tellin him lies bout me and tryin too get him back sayin she'll leave her fiancé for him. But the only reason I don't say anything too her is because she won't let us spend time with his son. I used too be best friends with her and was there since the day their son was born. And idk what too do.