There are ways to remove the yellow stain but it's not recommended unless you raelly know what you''re doing as you could potentially do more damage to the image as a whole. You can use a Permanganate/Hydrochloric acid re-halogenating bleach, that will often destroy the yellow stain. Then you expose to daylight and re-develop, fix, wash etc.
I spent some time in the early 1970's working on restoration of damaged images, some mouldy, you work with the least important first and evolve the best methodology, in this case it was harden the emulsion slowly.
The OP's wife's paper wasn't fixed properly, strangely it seems it becomes more of an issue when Rapid fixers become the norm. I'd refix in quite strong fixer, wash and selenium tone, then wash again.
Ian