Fc450 hydraulic clutch issue

I'm working on my friends bike after his clutch lost pressure on his 2nd ride on the bike. The master cylinder was empty so I know it's leaking after refilling multiple times and seeing it empty. I didn't see any leaks so we pulled the slave cylinder off and when you pull the lever it leaks around the plunger on the inside of slave cylinder. I'm assuming it should be self contained and this is the problem but I didn't see a rebuild kit for husky and the ktm brembo cylinder is half the cost so before we order the ktm version just making sure this is the fix. In the picture it leaks from under the black seal that surrounds the plunger.