I asked because it can be caused by an alergy and it can also have a psychelogical component . IF she isnt alergic to mushrooms the mushrooms themselves wont be a problem . If she doesnt know maybe she can make a pin prick on her skin and rub a piece of the mushrooms she would like to try into it . Then observe what happens . If she gets a swelling or other negative effects she should stay away from mushrooms . She could also try makeing a small amount of mushroom tea and droping some of that on a pin prick in her skin .
The thing about a posible psychelogical component is that if she is fixed on her problem , or it itches a lot she might have problems if she takes a trip .
The cortisone wont cause a problem and i think that the antihistamine shouldnt ....BUT i cant say that for definate . I do know that i used to have to take antihistamine and it didnt stop me takeing drugs of any sort .
So , be carefull , and wait to see if anyone else here has any information to offer .