I wouldn't make a definite decision on whether this mare is a frame carrier or not because I haven't seen pictures, but it sounds like it is indeed possible. Like I said, this white pattern gene is lethal in its homozygous form. If the foal inherits the gene from both parents, its colon will not be developed properly and it will colic to death within 72 hours of birth if not just put down.
Without meaning to sound intrusive, I hope she does know about Overo Lethal White Syndrome. It is not carried by Lusitanos, but since you said that might be wrong, here is a list of horses that carry it:
Horse/Pony breeds with Frame | Equine Color Genetics
It's terribly sad to have a mare foal a Lethal White foal when it is easily preventable. The test for it is available at UC Davis for $25. Frame is also very good at hiding, which is why the test is important. There is a mare on this forum that tested positive and only has a large star.