I can sympathize with her predicament - I just returned today from a 9-day Italy tour and one of my passengers' luggage was lost and never arrived at all. By the time she left Italy, they still hadn't tracked it down.
Elena Miro is your best bet. They make moderately-priced clothes, a little on the conservative side, but she'll find the basics. I don't recall ever seeing underwear there, though. For underwear, she could try the department stores, like La Rinascente or Coin.
The other plus-size option, but much more expensive, is Marina Rinaldi, which is the plus-size line of MaxMara (there are outlets, but not in the cities). In Rome, you can find Marina Rinaldi at Largo Goldoni 43 or Via Borgognona 6. Beautiful clothes, but pricey.