Terry's almost certainly got it right...
Blocked tank vent. The blockage may be in the vent thru-hull, or--especially if the tank is repeatedly overfilled--in the vent line connection to the tank, both the fitting and that end of the hose. A tank vent has two functions: 1. to provide an escape for air in the tank displaced by incoming tank contents, and 2. to provide a source of air to replace tank contents as they're pulled out. If the vent becomes blocked, flushing the toilet pressurizes it, creating back pressure that sends the waste back to the toilet. It also causes a pumpout or macerator pump to pull a vacuum after the small amount of air in the tank is exhausted, preventing anything more from being pulled out of the tank. If the deck pumpout fitting were opened while she's aboard and attempting to use the toilet against a blocked tank vent, there'd be a geyser out the deck fitting. But enough time is apparently elapsing between the time she leaves the boat and you come to pump it out for the pressure to escape via the toilet...lucky for you! Terry's article--except for the part about using a plunger (NEVER use a plunger in a marine toilet...the down-stroke won't hurt anything, but the up-stroke will mess up the seals, o-rings and valves in the pump)--provides all the instruction needed to clear the vent and then maintain it to keep it clear. Continuing to flush a toilet against a blocked vent can result in an eruption in the toilet or even a cracked tank. Strong suction against a blocked vent can implode the tank. So don't try to pump out the tank again until the vent has been cleared, and also instruct your client NOT to use the toilet again until it has been cleared. You might also want to check out the link below and suggest that she get a copy to learn how to operate and maintain her toilet tank properly.
