If the belt that drives the water pump broke, the engine would overheat while running. If she drove it overheating or didn't know it was overheating until it was too late, it could warp the cylinder head and blow the gasket.
When that happens either the engine will burn coolant (white smoke), coolant gets into the oil drain back ports and ends up in the crankcase, and/or engine oil ends up in the coolant system because there is pressurized oil that feeds the camshafts on the top of the cylinder head.
If it cracks a cylinder wall, you'll end up with combustion gasses in the coolant system, coolant in the oil, and/or the cylinder fills up with coolant.
If the head cracks, you could end up with coolant in the crankcase and if it cracks in the combustion chamber, you'll have coolant in the cylinder.
There is no telling what kind of damage is done until it is torn apart and inspected. When they did the headgasket they should have sent the head out for inspection and resurfacing.
Cracks in the block and cylinder walls are a little harder to find.
The coolant pouring out the bottom of the vehicle as your pouring it in usually means you have an external engine leak or the radiator/hoses let go.
There are no guarantees when it comes to repairing an overheated an engine. Overheating damages rotating and sliding components in the engine, so there is no telling what is going to happen or how the engine runs and sounds until you repair the leaks.
As for oil being in the coolant, it could be still having camshaft feed oil leaking in the coolant system or maybe even a broken transmission cooler inside the radiator.
Your mechanic should be able to figure it out if he/she is competent enough. If not, I suggest you get a second opinion at other shop.