I've bought a couple of guns from Walmart with mixed results. My first purchase went fine. The second taught me a valuable lesson.
I was young and less experienced. I didn't purchase it from my local store because it didn't have exactly what I was looking for. It was a stainless and polymer Marlin 22 mag bolt action. Store policy says that customers can't handle firearms without a trigger lock on them. Since they had to get the gun out of the back, it didn't have a lock on it, and I, therefore, couldn't hold it. The salesman didn't ever take it out of the translucent bag it was in and just did all the paperwork by looking through the bag. I naively trusted that everything was fine. Store policy also says that a happy customer isn't allowed to carry his/her just purchased firearm through the store, and it was carried through the door by a manager. I placed the rifle in the car and went back in to get some ammo (I don't remember if they wouldn't let me by it the first round, or if we were just getting it separate because my mon was getting the ammo). No problems with that. The rifle was a present so I didn't get to get it out and look at it for a couple of days. When I finally did, the bolt was very rusty. I knew it wouldn't do any good, but I took it back to the store. I went in first without the rifle and ask them about it. To my surprise, they told me to go get it. Although I, a happy purchaser, couldn't carry it out of the store, I, a disgruntled customer, could carry it back into the store without an escort. (Someone please explain the logic in that to me.) Walmart, of course, did nothing to correct the situation, and I had to send it to Marlin who treated me right. From that point on, I have never purchased a firearm without handling the actual gun to be purchased and inspecting it well. I learned my lesson.
I can't say that I won't ever buy another gun from walmart, but they aren't my first choice.
As to the original post, if they buyer wants it, has the money, passes the background check, and can legally purchase it, the clerk should sale it to her. I don't care if a 4' tall 80lbs woman wants to by a .50 cal barrett; you make the sale if that's what she wants. If asked for an opinion, then he could give it. I may even accept him politely offering it, but not refusing the sale because he didn't think it was the right gun for her.