So I had an argument the other day with someone at work. We were debating the mechanics of thrust in space.

I have always thought that throwing any sort of reaction mass out the back (opposite direction of travel) of a ship will add to (forward) velocity. A more powerful thruster will accelerate you faster, but in the end as long as I'm bolted or stuck to the ship with a magnet, I could throw pebbles from the rear of the ship and very slowly add velocity with each pebble toss.

My coworker (a LAAAAAAAAADY) contended that thrusters have a "max speed limit" and that (assuming the vacuum of space and no gravity) a thruster cannot accelerate anything beyond this speed limit (presumably the speed at which it throws reaction mass out).

Which one of us is correct?