I accidentally uninstalled one of my Factory App. What would I do to restore it?
I'd say your friend is making fun with you. If it's a system app (and there's usually a camera pre-installed that way), it cannot be uninstalled without root access (so you cannot have "accidentally uninstalled" those apps). He most likely disabled camera and gallery.


Go to Settings → Apps
Select the "All" tab (to list pre-installed apps as well)
Scroll the list to find your camera app. Check whether it is marked "disabled".

if so: tap that entry, hit the "Enable" button, done.

Repeat previous step for your gallery app