Hi Romano,
I think most cellphones that have a speaker will be suitable for her, but it's difficult to say without knowing the exact situation of her hearing. In some phones it's a bit tricky to turn on the speaker, so pay attention to that before you buy.
There are also some amplifing devices for cellphones -- for example one called CHAAMP that works with Nokia phones. Look it up in Google, maybe you'll be able to find a better combination than Nokia/CHAAMP.
