Since people from Tunisia need visa to enter Mexico the Italian visa is not valid for Tunisian citizens trying to enter to Mexico.
According to the Mexican Foreign Affairs office she would be able to enter without Mexican visa only if one of the following is true:
a) Holds a permanent residence of one of the following countries: Canada, USA, Japan, UK or any Schengen Area country
b) She has a valid American Visa
So, those are your only options if she wants to visit Mexico. I don't know if she is Italian resident or not; or if she holds an US visa; other than that she needs to apply for a Mexican visa.
I've been checking after the other answer was added and Mexican websites seem to differ according to which dependency you check. All national institutes seem to agree that only USA visas would be accepted, data seems to be outdated if we take in count the date the other answer states.
On Mexican consulates websites like the ones in San Francisco, New York and South Africa mention what the other answer says, that a valid visa from USA, Schecngen Area countries, Canada, UK and Japan is valid in Mexico. But others like the ones in Madrid, Milan and Roma only say that USA visa is valid.
Best thing would be to call the Mexican consulate in Italy and ask them, since they would have the real data.
Sources: Tourism Secretary Visa page (Spanish). Mexican Migration Institute: Countries that need visa to enter Mexico (Spanish)