Thanks sincerely for the feedback. I sincerely hope that the new cap works, but, if not, please don't hesitate to ask for help.
I do have a few comments that might help should the cap not fix the problem. Please take them as informational and not as second guessing your neighbor.
The gas cap isn't supposed to vent. It is designed to create an absolute and total barrier between the fill ippe and the outside world. I don't understand what your neighbor was trying to blow through, but I suspect there was a misunderstanding.
The code you referenced has to do with the "purge system". It consists of a solenoid activated valve that allows hydrocarbon molecules captured by the charcoal bed to be "purged" by being drawn into the engine's intake. Such a code could be triggered by a saturated charcoal bed, because the purge system would be unable to suck the fumes through the charcoal. Again, if the charcoal were damaged by the seafoam, that would cause a purge system code. I urge you to visit page 7 of the attached document to better understand the purge system.
A slight vacuum when the cap is removed just after shutting off the engine is not abnormal. A significant sucking sound is an indication that the tank cannot breath in as the gas is removed, and it breaths in through the charcoal canister. Again, that's a saturated bed symptom.
Again, I hope your new cap and your coathanger probing works. But if you still have problems, hopefully this information will help. Feel free to show it to your neighbor and discuss it with him.