Yes, 1/2" Insulation Board such as that made by Celotex is used. However, it's important to realize a couple of things (and what I'm about to say is situation-dependent. Remember, neither you nor I can see her house from here ...)
If the side of the house is uneven, then the installation of the board will only highlight the unevenness. Secondly, this 1/2" will make the home's Profile stick out, well, 1/2" more than it does now, PLUS the addition of the siding. The total could be 1".
Again, w/o seeing your situation, get 2 or 3 estimates and talk to a few Pros about your questions/concerns. Also, you can contact a mfgr. for 'advice' too. My best to ya and hope this helps.