The way I see it, she should have called off the wedding but thats spilt milk now yeah? Since she told a few people, her secret is a secret no more and it will come out eventually.
Its a very sad and potentially tragic situation. As a friend the best thing you can do is to tell her that her "secret" is going to be found out at one time or another so she should adjust and plan accordingly. Leave it to her to decide how she wants to handle it. But perhaps let her know that the real father of her child has a right to know about his child and her husband has every right to know that it isnt his.
The thing is, perhaps her marriage is happy. But her dark secret is known and will become known by more people over time. Does she want to let this thing fester or better to face it straight away. The way I see it, her very life could be in danger here so she needs to think very carefully about what to do and how to handle it.
What she said.