What he is saying is the cured the symptoms by doing a bodge and did nothing to fix the problem.
When you hit the brake/clutch a tensioning pulley moves off the drive belt to allow it to go slack enough to ride on the engine pulley and not be driven by it.
So instead of getting the correct amount of slack in the belt he hammered the sides of the pulley apart thus making the drive diameter smaller so the belt then had enough slack to slip.
Remove the battery , drain the fuel and slip the deck out.
Hoist the mower up till it is standing on the back so you have a clear view of what is going on.
Alternatively you can put the mower on tall axel stands but it is a lot easier to see what is happening & work on the mower standing up than laying under it constantly getting dirt & muck in your face.
Reach around the front and work the clutch/brake pedal and you will see a small tensioning pulley moving back & forth it is usually plastic and about 4" diameter.
To work properly this pulley must allow enough slack in the drive belt to allow the drive belt to slip freely on the engine pulley.
Check all of the links & pins you will most likely some flogged out which prevent the tensioning pulley moving far enough or accumulations of grass and dirt peventing them from moving fully.
On most mowers there is an adjustment to alter the distance the tensioning pulley moves.
There will be another adjustment on the brake itself which might need tightening a bit to increase the braking force applied to the mower