Hi need2relate,

Glad to have you here with us. I searched around a bit to see if there was anything that would speak to your concerns. What stuck out for me was this on the
Do's and Dont's of a BPD Relationship:
https://bpdfamily.com/message_board/index.php?topic=62266.0Even with the best of intentions and the highest of hopes there are no guarantees with any relationships. Your compassion, understanding, and joy for life are great assets for making a difficult relationship succeed. Your desire to set a firm line on cheating is an understandable one. (<---just my opinions.

) Does she have a firm understanding herself of how/why it has happened? Does has she have a willingness to not do so again?