The one on the firewall where you checked the fluid level is the *master* cylinder, the slave cylinder is down on the bell housing near the clutch. Look there and you will probably find some leakage.
If your friend will just push the clutch down with the engine off, if the slave cylinder is leaking she will find the pressure against the clutch pedal will gradually reduce.
If you do this test a couple of times with some newspaper under the slave cylinder, you will probably notice leakage.
If there is *no* leakage from the system but the pedal pressure drops off, the problem is probably an internal leak (or leak into the foot well) by the *master* cylinder.
What to do depends on the diagnosis, and how critical it is for your friend to keep this car on the road.
Welcome to the forum BTW, it would help if we knew at least the region you are in (humid or dry) as that has an effect on brake and clutch hydraulics.