Assuming that the coolant loss was not external, your brother would have been wise to follow the advice you have received here.
Please take time to read this: http://www.bonnevilleclub.com/forum/...e=article&k=38
Adding stop leak to the coolant in an attempt to slow or stop an internal leak caused by either a perforated Upper Intake Manifold or failed Lower Intake Manifold gaskets in these engines is foolish. Quick action (as you have been advised) might save this engine for $250. Continued driving will almost certainly destroy the engine.
Certain detergent additives added to the oil can free a sticking lifter and stop a lifter tick under certain circumstances. Thick oils and viscosity improvers like STP may quiet a very light bearing knock temporarily.
But no additive can replace damaged or worn bearings, nor will they do anything to help an internal coolant loss caused by the most common failures described in the link above.