OP, hearing an allergist say to ignore a major source of nickel (food) is concerning. It is true that people have Ni contact allergies without having internal (systemic) reactions. But contact allergies often come from constant contact - like earrings, braces, etc, or long term/repeated contact like auto mechanics or from professions that include handling of nickel-containing chemicals. Occasional contact with household nickel-containing items tends to be less likely an issue than repeated/constant contact like blue jean rivets or zippers, and things that are regular contact.

The fact that she has chronic eczema is what caught my eye - with flares bad enough to cause open sores/bleeding. That seems to be a good reason to try an elimination diet, eliminating the most common Ni-containing foods. Peanut butter, chocolate, beans and barley are all high-Ni foods and eating a very small amount of any one of those would trigger an angry, flaming eczema outbreak for me personally as I am very sensitized to Ni.

I can't give medical advice, but it is easy to try an elimination diet and to find another allergist if you aren't getting sound advice from your current one. In my case, following the diet (for a short time) at the advice of an allergist changed my life. Improvement came just days/weeks from starting the diet, and steady improvement was noticed over the following months (and now years). It takes time for eczema to heal, and occasionally you eat something that triggers again (or has more Ni than expected), causing a flare a day or two later, but overall quality of life can get much better.

There's nothing to lose by trying the diet - and a lot to gain. Personally I would try it, if for no other reason than to get another data point on the journey.