If it's just bits and pieces she will need it for then even if some places do take Euros then I imagine it would be only larger chain like places, hotels, large tourist attractions etc... Many lovely things can be found from street vendors or small local shops and markets, similarly small cafes for coffee, and I can't imagine many of them would take Euros.
Even if they did then there is a change issue. Unless she managed to have the exact amount all the time I imagine change may be given in the local currency (even if they take Euros, doesn't mean they have change in Euros) and it could become a nightmare trying to then work between three currencies in each country.
It would be awful her her to find something nice to buy or somewhere to stop for coffee and for her to be unable to buy anything as she only has Euros.
Also bear in mind that, unlike Prague, English is much less widely spoken (well at least) in Hungary, even in Budapest when out of hotels/tourist restaurants. It may not be that easy for her to broach issues of currency in some, especially smaller, places.