Stand your ground. If she's like my X, the more times you reject her notion date or even be friends while OM is around, she'll eventually deep six him if R is what she REALLY wants.
Me 35/XW 33 S13 & S12 M: 10/17/98 OM & S: 07/08 D final 06/09/09 12/03/09 - 06/13/10 "Piercing" 06/13/10: Engaged to Re-marry 10/17/10 06/25/10: Expecting baby #3 2/14/11