I have never had problems with keeping another adult with a mother and babies, but it does depend on their temprements. What she could do is put the mum to be and one of the others in the new pen, to await the birth so she has company, then if after the babies are born there seems to be any problem she could seperate them straight away. That way the pregnant one will always have company and shouldn't become stressed.
I have 2 pregnant sows at the moment due any day. They are sisters and always get pregnant and have their babies within days of each other, but I also have an old spinster pig who often goes in with a pregnant sow to keep her company, she seems to enjoy playing auntie, and only gets a bit grumpy as the babies get bigger and are close to being weaned.
it may be a case of seeing how it goes.