I would like to say she wouldn't and she would actually have additional protection against pregnancy/maternity related discrimination because the employer is aware of the pregnancy but I've seen too much shitty behaviour to be confident that would be the case.
It is also never as simple as just making a tribunal claim because the amount paid out for damages due to discrimination is not much and is tired according to the bands determined by the Vento case which have only increased once (I think!) is last few years and something like withdrawing a job offer due to pregnancy is probably going to be in the middle band which was max £18k a year ago (I'm just going back to work so a bit rusty) and she'd be lucky to win without substantial proof never mind get much of a payout. Very few cases get a large payout. The process is time consuming, long and stressful.
Other factors are at play too. It might be that she works in a small industry or there are very few employers and taking action can blacklist people in some cases (it shouldn't unless the claimant is malicious) and it would be naïve to believe otherwise.
But mainly because some people are astonishingly ignorant of the law and make stupid decisions which are then fairly easy to cover up. Job withdrawn due to maternity discrimination? Oh no, it was because the referencing wasn't ok/the funding has been withdrawn/the budget has been reallocated/an internal candidate applied who has done the job before and scored more highly. Or my personal favourite "the job never existed in the first place. It was advertised and recruited in error".
The trouble with pregnancy/maternity discrimination is that you have to prove it is pregnancy/maternity discrimination not just general incompetency or a mistake or a valid business reason. Tricky.