my mechanic noticed a dime size piece missing off my front rotor while doing some work on my durango.?

First question I'd have is where is the dime sized missing? If its on a spot that doesn't contact the pads there's probably nothing to panic about. If it is then its probably a good idea to replace it but I doubt that there's any real danger of the brakes locking up or failing because of it. If the piece is missing now that means it has already fallen off and you've been driving it since it did with no issues so why would it be now? Its always best to replace rotors in pairs to keep your braking action even on both sides.

Rotors are cast, not machined, so it's quite possible to see a small piece break out of one and depending on where it is (on the surface or on the outside diameter) it might not hurt a thing, unless there is a noticeable crack in the rotor. THEN it should be replaced. Keep in mind, there are rotors out there that are drilled and slotted so they will run cooler, so a surface imperfection is not a bad thing if the brakes are okay otherwise.

Replace them both. They are not very expensive and are easy to install. If a big piece breaks off it could cause the brake to lock up.

Well if the pad got snagged on the bad spot it could lock that wheel up. Causing control issues and more damage.

that rotor needs replaced asap. it will crack and start ruining your pads, etc.