Altitude sickness Mt. Etna

She should ask her primary care physician or some other medical professional.

Mt Etna is almost twice as high as Denver. It’s the two mile high volcano.
But maybe it was the bad food of Colorado that did it. Sicilian food is definitely better.

Denver is altitude 1609 metres a.s.l.
The TOP of mount Etna is 3357m a.s.l. but the Crateri Silvestri are at 1900m a.s.l.
What was the problem in Denver and how long did she stay? Is there some medical reason or does she just feel bad?
If there is no specific medical reason which means it is inadvisable, if you are just going on an organized half-day or day trip to see the Craters she'll probably be back down before she starts feeling the height. Try not to stay up too long, and to not do anything too strenuous. And asking your doctor is the best idea.

I would suggest talking to a medical professional. If she had trouble in Denver at 5276 ft., how is she going to tolerate the flight to Italy?? Airline cabins are only pressurized to 6000 to 8000 ft. Maybe it was something other than the altitude in Denver.

We are looking at a tour that would only take us up as high as the Silvestri Craters which are not all the way to the top.

The thing to consider that most people do not when visiting Denver, it's very dry, and being dehydrated can aggravate altitude sickness. Drinking a lot of water, and perhaps taking aspirin before going up the mountain will help.

As with any type of altitude increase, it's best to acclimate slowly, gradually going higher. We did this in Peru before walking the Inca trail, and we are from Denver... But if you have just one day to make this trip, that won't be possible.
That said, some people are flat landers and can't handle our altitude !

PS, we have great food here, come try our Rocky Mtn Oysters...