I think you're right to be cautious. Problem is, it's hard to know if there was a good reason for her to become protective (reasonably perceived threat), or not. Yes, a shy or snappish dog very likely WOULD pass those traits on to offspring (temperament is as inheritable as looks and other genetic factors) - and depending on what it was combined with (sire's temperament), could produce something very unfortunate.
It may be a little unfair to judge on this basis alone, but the fact that the sire is on site also sends up a little red flag. Most often that suggests a backyard breeder who has mated the two dogs readily available, rather than seeking out the best match for an evaluated bitch. It isn't always the case - but very very frequently it is. You're the one in the best position to determine (i.e. ask) the reasons behind this particular pairing though
As above, it is also critical to see BOTH parents' test results for the screening for cardiomyopathy, aortic stenosis, hip dysplasia and hypothyroidism before making any decisions about a puppy.