Unless there was a catering error, the shrimp come served in a self-contained ramekin not touching any other food (but sitting atop a garnish). The ramekin is sitting next to the salad on a longer plate. Depending on the severity of her allergy, I can understand the concern of cross-contamination from one of the caterers plating the shrimp and touching the outside of the ramekin, and the ramekin touching the salad, but there's not a whole lot you can do about it at that point. As noted above, all of that comes pre-plated, so all of the trays with the shrimp have the same limitation. I'm sure the FA would have taken the whole salad plate off the tray if she had handed it back to them, but short of that the option is pretty much not to eat it.

In fairness to AA and the caterers though, they do serve the shrimp separately in their own container so that people that have issues with seafood can still have the salad as a separate course in most cases. There will always be edge cases though that they can't design for, and that little shrimp appetizer has been one of the most popular features in AA's catering since hot meals were restored.