Unfortunately, this is one of those things where
actual risk may be less important to a family than "PROTOCOL" is.

I think that we can all agree that the
actual risk, as you've described it, is probably quite small. Definitely into 'traces of traces' territory. But please understand that for FA children (and by extension, their families),
procedure is sometimes important for no other reason than that following those rules even when they are inconvenient may one day be all that stands between that person and death. It's about maintaining a margin for
some human error, YK?
It's not that you're
wrong, necessarily, to see this as extremely remote in terms of actual risk. It is. BUT-- if your BF and his DD have a rule about this kind of thing, well, then... the rule is the rule. (Calling and telling her not to eat the sandwich seems a little much, but a lot depends on age and reaction history.... risk here ranges from "veryveryveryvery remote" to "unlikely" and I might call, too, with "unlikely")
This would violate our house rules-- which is why we would probably pitch the margarine at our house.
We have a rule about no double-dipping in any condiments or spreads, and this is why. A
single dip with the knife is fine, but if you need more, at our house, you wash the knife or use a clean one.
Another weird/seemingly over-the-top rule at our house is that food which has been OUTSIDE OF THE HOUSE (at a potluck or even just at a friend's house) is
always "contaminated." Even if we are pretty much 100% certain that it isn't, the allergic person never touches it again.