I agree with calling the friend as well. I'm not in favor of taking old soils or patching the fix unless it is truly confined to a small area (normally around a drain pipe.

If the budget allows, and if the source of the leak is in that 2' deep add-on section. I'd suggest getting a track loader and vibratory sheepsfoot in. Source good, fresh clay from a borrow pit, drain the pond a few feet (a foot below the projected excavation depth) and re-core about 4' deep, blend in the back slope with the removed core and recover/seed topsoil. Given the pond size, cost and time should not be oppressive and your piece of mind is as priceless as looking out over a full pool!