Aloha! I don't think any boats out here will allow anyone that is pregnant. While we ("Explore Kauai Scuba") allow snorkelers to come along, we don't allow anyone that is pregnant on our boat either. Not only does our insurance not allow it, it's just not worth a simple loss of balance and a fall on the deck to make a fun day go bad quickly. Funny story, I've had a lady trying to hide the "surprise" until the perfect time, well that ended up being dock side that morning when she whispered her secret to me, she had to spill the beans to hubby! SURPRISE! They were cute and happily went to breakfast to celebrate instead.
Like diversteve said, Koloa Landing would be a great location to get a dive in and she could easily slip in if conditions are mellow, which in January usually are super nice. I would still suggest her going in with a buddy though for safety reasons(slippery entrance, etc).
Let me know if you have any further questions, I'd be glad to help.