While I was driving the engine began sounding really loud and rough

This is hard to figure out without actually looking at the vehicle however, based on what you are saying this mechanic found, the timing belt most likely jumped timing and allowed the pistons to hit the valves. This will bend the valves and possibly the piston connecting rods.

So if the engine is knocking loudly and it happened all of a sudden I suspect this is what happened. It may be worth while to take it to another shop so someone can put their hands on it.

However, if you want to try and figure this out we need to use a borescope to look down in the spark plug hole to see if there cylinder walls are damaged and if the pistons have marks on the tops of them from where they hit the valves.

Let me know if you have questions. Thanks