Sounds like the coil spring that operates the rotary magazine spool has lost it tension (or, worse, broken)
The magazines have a reputation for being tricky to work on because of an adjustment process that almost needs 3 hands to accomplish (having a grasp on how they operate and how the various parts interact is extremely helpful too)
There are a few descriptions on the internet and only a couple YouTube videos on these and how to correctly adjust/repair them if he wants to attempt it himself (I would)
No idea on where to get parts if he needs them but I'm sure they're out there, they made a bunch of those 99's and it doesn't appear that there were too many revisions of the internals over the years
I would suggest Numerich but I heard a rumor that they are currently not in operation.
I'm
FAR from an expert (I do own a 99E in .308) and I know it's not very specific info but I hope it helps
Setting the Rotor on the Savage 99
Edit: don't know if it worked (never tried to post a video before but this might be of some help.