But you know if you don't that it could hurt this person, their grandchild and a few others as well.
If she is in physical danger, or financial danger, then she should be told. Nobody should remain in danger for any reason.
If it's feelings that might be hurt, then perhaps take a giant step back and stay out of it. Other people's relationships are for them to work out.
Your details are sketchy, but that's how I see it.
Hugs