First, I know of few people that would ever say a foal is stillborn. They can have, say, a twisted cord and die in utero, and then be delivered dead, but I don't know of any that were just born as a stillborn foal except in Rh factors.
I have heard, but not experienced, the death of foals sometimes with repeated breedings to a particular stallion, when there is a Rh factor blood type incompatibility. Just like in humans that have a negative blood type, carrying a positive blood type baby. My daughter, for example, is A- blood type, so she receives a shot for Rh incompatibility. If she is bred to the same stallion, in that case, and offspring could be a stillborn, and die in utero.
I could be wrong, and ask others to correct me. She may have also had a difficult delivery, with no attendance at birth, and the baby be found dead and the owner call it a stillborn. However, this breeder is showing a horse as "sold" that I know the horse actually died. But apparently rather than admit it, she marked the horse sold -- as a friend had an interest in it, and was told the horse died. So, with this breeder, it's a bit confusing when talking truth.
But as I say, I know this mares breeder, and she breeds quality, not quantity, so, I'd probably still take a chance.
But perhaps our friends here could render an opinion. If she's pregnant for a spring foal, traveling would be safe at this point for her.