as you suggest, we often don't realize they are there until something unfortunate happens.
Did your male Betta show anything leaving the body which would indicate internal parasites?
Was that Betta becoming emaciated? That could indicate Hexamida, but if he was a year or two or three old, it could also just have been aging. Aging is usually not the case unless it is an older fish (for the species) and so that too can be a rather arbitrary explanation for a death.
But it does happen. And it is more likely with fish which have been correctly and thoughtfully cared for. I would guess that in the aquariums of experienced aquarists, a greater percentage of Bettas and guppies die of old age than in a population of wild fish.
If you still have misgivings about your female, then administer one of the standard treatments for internal parasites. Your LFS should have one for sale. (I know, that old "cha-ching!" factor again.) That sometimes is worth the money for the conscientious aquarist, just to know that the fish is safer company for others.
And then you have that treatment for whoever you purchase and quarantine.
All the best!
unclescott