I'm so sorry your friend is in this situation. She's very lucky to have such good friends like you and your partner.
The more you and your partner understands about DV then the more you can stabilise your own selves for the long haul. You know already that it can be a very LONG haul.
One of the things DV abusers do is "isolate" their victims. Many victims, like myself, have everyone they know turned against them by the abuser(s). It's ok we're well out now.
Having no one to turn to is one reason why victims stay.
It will be emotionally challenging for you both to KNOW what's going on and to hang in there for her and the baby. But it seems you are very committed and I acknowledge your convictions.
Police told me recently to "just call us" when I knew a violent situation was occurring in another household. You can do this if you know. You can also speak to her Local Police Social Worker and tell them about the situation. Ofcourse SHE will have to be the one to report him etc but YOU can give Police the headsup.
She will make every excuse known to skip meeting up with you etc. Keep contact.
Abusers may also be recording calls, going through texts and download all sorts of apps to monitor their victim's phone. Try to take her out places and encourage her to open up to you. You have the perfect excuse now she's pregnant to just turn up with "something for the baby".
Local Women's Health Centres run incredible courses for women in DV relationships and those recovering from them. They never give details out to spouses, so her confidentiality is maintained. She can say she's going to a Mother's group which is pretty close to the truth lol and attend one of these courses even with the baby.
This was a pinnacle point in my understanding of DV and gave me more of my strong conviction to leave for good.
Best wishes. My Prayers are with you all during this time.