Mate try this

Run a clear or somewhat translucent tube from the nipple to a bottle half filled with fresh brake fluid- make sure the end is BELOW the surface.
Open the nipple with a 8mm spanner , open the clutch reservoir cap and start pumping the clutch pedal by hand. Get
Your mate to monitor the reservoir and top up - keep pumping until a steady stream of fresh fluid fills the bottle. Tighten nipple, remove tube and top up the reservoir if need be. That should be it.