Just Bento

I am not sure what your mother is allergic to, if it is just shiitake mushrooms I would suggest another type of mushroom. But if she has a mold allergy, where she cannot eat cheese or any type of mushroom, then I do not know of a replacement.

hm, difficult. maybe some frozen firm tofu for texture and as a filler? or seitan.. it's got a similar kind of chewiness like mushrooms and takes up the flavour of the soup liquid nicely.
bits of veal might be similar, too, if you're not a vegetarian.
if your mother can eat any other mushrooms, just take those. i'd maybe fry them a bit first if they're fresh to intensify the taste, as shiitake has a rather strong flavor..
what else goes into the soup?

I'd be interested in this as well. I don't eat mushrooms as they make me ill and I'm slightly lactose intolerant as well. What substitutions would be good for soups and/or recipes that call for mushrooms?

Mushrooms are packed with umami, so the best substitute for them might actually be meat! If mushrooms are called for in a sauté or in a soup or stew as a supplementary item, just leave them out. In a stew for instance just add a bit more of the main meat. To make up for the umami, besides meat you can try adding (if it's a Japanese dish) more konbu seaweed, or (if it's a Western dish) umami-rich vegetables like celery and onion.

Of course, you'd want to avoid things where mushrooms are the star ingredient, like mushroom soup, because you know...it's not logical. Kind of like trying to make roast beef with a beef substitute. ^_^