Generally silver-allergies are because of the nickel in the alloy. Pure silver is hypoallergenic. Source: My wife has this allergy and all her silver jewelry is sterling to avoid the problem.

So one can imagine the werewolf protagonist saying "Aww sorry babe, but I can't handle silver, I'm allergic", and their conscientous loving partner goes and does their due-dilligence and gets a pure-silver ring which shouldn't trigger the allergy..
Except it's not an allergy, it's a supernatural aversion to silver itself, and now they've got no explanation.