Clutch stiff on release

I've just driven my GF's mom's 2009 4 cylinder, 5 speed manual, regular cab. It has only 27K mi.

I noticed that the clutch is really stiff on release, meaning that it's slow and resistant returning to the off position from being pressed. Much more so than on my 2007, v6, 6 speed manual, which pops right back. But I replaced my clutch master cylinder last year. My truck also has 100k mi more than hers.

I checked the brake fluid level and it's okay. Could it be that she needs a master cylinder replacement as well or something else?