Not Italy, but I and my immediate family have bought and sold several properties in France - plus three "near misses" where the properties had hidden defects and we had to extricate ourselves from the purchase. Looking at this article, the Italian system seems to be a mixture of the French and UK ones: http://www.detulliolawfirm.com/property-selling-guide/
I've found the most important thing is to get a good bilingual property lawyer (although my French is pretty good). The one I use currently charges EUR 300 an hour and is worth every penny. He has saved me many times over what I've paid him in fees. Even if her Italian is perfect, it would probably be helpful to your friend to use someone who is used to dealing with non-residents. It is no problem being out of the country when the sale goes through, either - you simply give power of attorney to your lawyer to sign the contract for you.
There aren't any 'budget' estate agents in France that I know of (the sector is heavily regulated), so I'd be surprised if these exist in Italy. If her BIL can show people around, why can't he let the local estate agents in to give a valuation?