This posting was unbelievably apropos. A well established patient of mine, with celtic skin, had had numerous AK's over the years treated by me. I finally convinced him to try a course of 5FU about 6 weeks ago. His reaction was as to be expected and quite severe after three weeks. Well, apparently it continued to get worse.
Allergic reactions are extraordinarily rare.
Of course, your nurse may be the rare bird. However, my hunch is that retin-A is irritating while the Renova is not. Perhaps he is allergic to the steroid. If so, with continued use of Westcort, he'll probably start reacting to that.