I'm not suggesting a rescue mission or anything like that, but if she were able to get to a safe place, she should be able to return, for at least two reasons:
1. She was 15 when she went there - most places don't consider 15-year-olds to be as fully responsible. That she was able to do what she did at 15 speaks poorly of the UK's child protective services generally (and from what little I've read, they're disgustingly poor at protecting children if doing so might involve interfering with proclaimed religious or cultural beliefs?).
2. She has a child who is or I assume has a claim to UK citizenship - if only for the child's sake, she should be allowed to return.
3. What happens when she returns, whether she is a fit mother or not, etc. are other considerations.