Oh what a nightmare. I've seen situations like this before, and as a cashier, I used to try to step in and ask the next customer in line to back off while the current customer finished their transaction. Luckily it didn't happen a lot at our store, and we never had an issue quite that bad.
It's sad that I could take three larger dogs into the pet store and not have an issue, yet one lady with a freaking Chihuahua can cause that kind of trouble. I used to bring them in on my own, and never once had an issue - and only one of the dogs ever went to any kind of classes - I trained them otherwise.
I also walked them around the neighborhood on my own regularly. Since I didn't have a fenced yard at the time, I would have them on tie-outs at home, but to keep them mentally active, I walked them a couple times a day at least as well.
It didn't even take all that much work really to teach them the basics of what I expected, and yet, they were better behaved than many of the dogs I met in our area. For example, before playtime in the snow, I had them wait until I told them they could go ahead and play, and they would.
After I turned them loose in the snow, they had a total blast, and would just chill in the snow for as long as I would let them.
Hopefully you won't run into any more idiots like that. I wouldn't be too worried about your pup, it sounds to me like he did pretty much perfect at his job, and if he'd have wanted to he certainly could have hurt that little thing a whole lot.