my coworker took the car to her mechanic twice and he is saying nothing is wrong. the problem allegedly happens when she drives the car all day, not intermittedly.

Sounds like a defective master cylinder or the power brake booster. She should find a competent mechanic.

check the brake pads for wear and the brake fluid level.
also have your mechanic check if you have air in your brake lines and have them bleed the lines if necessary.
if all are ok, chances are good that it is a defective master cylinder. (this should be the last resort since it is most expensive to fix)
but a good mechanic should already know this. so find another mechanic who'd be more helpful

check the brake fluid level on the reservoir, if its losing fluid there is a leak somewhere on the line system. try to push the pedal hard and leave your foot there and if it goes down beyound where it should be, the gasket of the master cylinder is broken because the fluid is escaping between the gasket and cylinder in slow pressure of your foot.