The health certificate she's getting for him basically certifies that he was disease free at the time of exam. The vet won't have done any lameness checks, looked at teeth, eyes, etc. It's definitely a good idea to have a vet of your choosing do a pre-purchase exam before the sale is final.
Since you already know the seller, see if she will let you take him on a trial (at least 2 weeks, preferably 30 days) to get the pre-purchase exam done by your local vet and determine if he's really a good match for you. If she's truly interested in making sure he goes to a good home and thinks he would be a good horse for you, she shouldn't mind doing a trial period.