Well there is at least in your mind another side. There are enough facts in place, to place this pregnancy at a high risk. Let us review them.
1. Unattended female outside - owner admits she has seen at least one dog in the yard. ... .. Means at any time other dogs can get inside the yard too.
2. Witnessed "one" dog going after her female.
3. Her dog is NOT health tested and prepared if indeed she should even bring puppies into this would - is presumably mated with some-one at some time. Or more than someone at various days n times.
4. De facto who ever the sire or sires are ; are of un known size and of unknown health, and of unknown health lineage
So given all of this; why would you not consider you are putting this female at a huge health risk to even go forward with a pregnancy.
Then if she successfully whelps what-ever pups she whelps, is the owner financially able not to mention experientally able to nuture and bring these pups to a healthy place? Then how will this owner find homes, and does this owner of this dog have the funds to provide a health guarantee against genetic defects of these puppies?
Sobering thought. I wish all bybers and "accidental" breeders would be made to be financially responsible for all the health woes they foist onto unsuspecting and naïve buyers.