It is possible. I am petite but was under 100lbs when I started to try for a baby. I started eating whole milk (something I'd read somewhere) and gained a few pounds. I think it's entirely possible that my body said, "Nope!" til I weighed more and was able to carry a child. Very very thin people do get pregnant, yes. But if she is that tiny, it's something she needs to discuss with her OB/GYN. She may be malnourished as well as just being really thin. (In my case, I was naturally thin and also power walked a mile or more daily chasing trains on my commute.)
She may have other factors, too. If she hasn't seen an RE or had a workup to find out the cause of her miscarriages, she should. There is a condition women can get where they cannot carry a child to term. I forget what it's called, but my friend had 2 healthy babies and then lost three in the second trimester. They finally figured it out, treated her, and she had another daughter. It only happens to women after they've had a child or two.