First, separate the theoretical/fantasy from reality. Sure, in fantasyland, you ride in on the white horse rescue her from the stupid ass boyfriend, she looks into your husband's eyes and falls madly in love and everyone rides off into the sunset together. Nothing wrong with that fantasy as long as you both realize it has very little to do with reality.
Almost guaranteed anyone she dates after current boyfriend is going to be a rebound relationship. Do you really want that to be you guys? She sounds like she needs supportive friends, not someone who's long term agenda is to get into her pants. Be her friend first.
A word of advice: two women sharing the same living space is difficult under the best circumstances, add in a pregnancy/post partum AND a poly dynamic, you are asking for huge amounts of drama that will likely end in catastrophic disaster.