My mother is Bi-polar.
I grew up in her house, left when I was 15 - it's a miracle I turned out fine (stable, good job, etc) my three siblings, all younger than me, weren't so lucky....
I've done a lot of research. I'd recommend a book called "An Unquiet Mind" by Dr Kay Jamison.
From previous answers, there is disagreement on Violence. My two cents:
Bi-Polar is a series of emotional inabilites to handle stress in life, and certain behaviors are "explained" by this lable. However, there is not sufficient scientific data to determine if there is a chemical cause. It IS known that medication COMBINED with psychotherapy can alleviate some problems
MEANING: It's TREATABLE - whether or not the Bi-Polar is a factor (note: NOT Excuse) then he is not being treated. And your daughter is trying to make excuses for his behavior, which he doesn't want to change.
I would recommend you contact a local Domestic Violence Advocate, and get advice from them. At this point, your concern should be for her and not for any excuse he may be using. It is a fact that violent men become worse during pregnancy and after a child is born. You should be very concerned for the childs safety, but get professional advice before talking further with your daughter.
If you approach your daughter about his behavior, she may stop talking to you. You need a talking point, and to let her know that you are willing to do what it would take to get her to safety. And know that at this point, you are no longer safety because he knows you, too.
I left my husband the first time he laid a hand on me... I didn't realize I was already pregnant with our second at the time, our first was six months old.
Good luck.