Can a leaking water pump ruin a timing belt?

Yes if you have a timing belt driven water pump this is very possible. Alex

Strongly Agree with Alex and your Mechanic because mechanic has to remove the timing belt from the water pump pulley to change the pump anyways. It's always suggested to change the belt and the pump at the same time. Pump is timing belt driven. It is always recommended to replace the timing belt with the replacement of the water pump. you don't want to do this job twice! it all comes in a kit together,less than $200.00,and you have an Interference Engine,so you don't want the belt to EVER break!

It definitely will. It may not seem so at first, but it will deteriorate over time