Garmin vector based maps (.img files) will work on the Nuvi 2797, however the default display of topo maps on the Nuvi is pretty ugly. Contour lines are fatter than the roads and clutter the screen, many POIs will not show, wetland areas look like lakes, etc. It is possible for a map author to improve the appearance of maps on the nuvi, and I have spent quite a lot of time on this myself. However, I am not aware of anyone else on this site doing this.
The bottom line is that you will have to be the judge of whether this is acceptable. Download a map that you like here - see this tutorial:
http://www.gpsfiledepot.com/tutorials/how-to-open-maps-in-garmin-basecamp/Now get a micro SD card and place it in either a USB reader or card slot in your computer. Follow these steps to send the map to the card (Garmin's software will treat the card the same as an actual GPS):
http://www.gpsfiledepot.com/tutorials/how-to-load-maps-on-my-garmin-gps-unit/You should now see a folder named Garmin on the card with a file name gmapsupp.img inside. Change the name of the folder to Map. Bring it to your friend, turn off his Nuvi, insert the card and restart the Nuvi. Press Settings > Maps and Vehicle > MyMaps. Disable the City Navigator Map and enable the GPSFileDepot topo map. Now you can decide if you like the results.
OTOH, after thousands of dollars spent on Garmin devices and thousands of downloads of my maps here at GPSFileDepot, I'm starting to think the Android platform may be a better option. I have a Dezl 760 which has the same screen and hardware as the Nuvi 2797. The Samsung Galaxy Tab 4 8.0 that I just got has a bigger and much brighter screen, a faster processor and 5x longer runtime on battery. And it was cheaper. See:
http://forums.gpsfiledepot.com/index.php?topic=3898.0