Most pressure canners (all that I have ever seen) are called "Pressure Cooker/Canners" - or - "Pressure Canner/Cookers" ..... so yes - they are dual function.
Just tell her to read and follow the instructions that came with it ..... basically, never fill the pot more than 1/2 - 2/3 full. She can obviously get more "chickens in the pot" if she cuts them into quarters, or 8 serving pieces, than trying to stick in whole chickens. The main thing isn't how much is in the bottom of the pot .. it's maintaining the headspace at the top of the pot so that pressure can develope, and not clogging the safety valves.
All-American makes several sizes ranging from about 10.5 - 41.5 qts. If your DIL doesn't have the instrustion book it would be a wise investment for her to get one (beats having shrapnel flying around the kitchen). One source for a replacement instruction book is: http://www.stonesmerch.com/pressurecanner6.html
- or you can get them directly from the maker (Wisconsin Aluminum Foundry) at: https://store.wafco.com/ezmerchant/home.nsf