Was this a valid concern?
It should be a GREAT concern. Least of all simply because of the individual's freedom to choose and consume a snack of his own preference.
More importantly, though, one has to question whether the member of cabin crew is fit to be doing that job, in that environment.
IF (and it would need verifying) she was so allergic to nuts that it was justifiable for her to ask a passenger not to consume them, then she might be at risk of a major anaphylactic reaction when in close proximity to nuts. This is a potentially life-threatening emergency, and ample grounds for an in-flight diversion. What if the aircraft was over the mid-Atlantic ?
IF it were true, that individual should not be doing that job. If she were NOT that allergic, then why has she bothered mentioning it at all ? Mere proximity is unlikely to be a problem, only if she ATE any herself.