The 3.5L engine in that vehicle is an interference engine. This means that the pistons and valves share the same space in the cylinder. When the timing belt breaks, the valves are no longer synchronized with the piston movement. As the pistons continue their travel, they'll likely collide with the open valves, bending or breaking them. This can also cause damage to the piston(s), cylinder head, and connecting rods. The extent of the damage will vary depending on the precise position of the pistons and valves at the moment of the break.
Driving at 65 mph adds to the severity. The engine is under load, and the sudden loss of synchronization will result in more forceful collisions between the pistons and valves. The inertia of the engine's moving parts at that speed magnifies the impact.
In short, a broken timing belt at that speed will almost certainly require a major engine repair or even a complete engine replacement. It's not a simple belt replacement fix.
