If he checks and it's overfull, that typically means the pump isn't pumping. Assuming someone didn't dump an extra X quarts into the trans w/o checking fluid level. When the pump doens't pump.. the level is higher than when it's pumping, hence why it appears overfull. If the pump isn't pumping.. there's no fluid moving. My meaning here for a guy that doesn't know what the trans does and how it works is generic. Most of my posts are usually easy tests etc that a guy w/o any real equipment can check quickly.
The fluid out the breather is commonly to a channel plate gasket. This will usually mean pressure is bleeding off.
Also, I'm aiming at what most guys typically see failures from and what I've encountered and read up on. It's not perfect and from the sounds of it you have a ton of experience with transmissions.
