your friend's experience but I think the most important one in dealing outside the authorized dealer network is to always assume the downside and be prepared to deal with it.
Everything you described (strap and chrono problem) could have occurred while in the possession of the original dealer or the second hand dealer. Hard to know but obviously someone tried to hide it in the safety of the sealed sleeve.
When on the hunt for something special, it's far too easy to overlook some things because "the" piece is within your grasp. Afterwards, when you discover something ... you feel like someone took advantage of you, particularly when you paid a premium. Which is why I always take things like this into consideration and always assume there may be some hidden issues. I feel confident your friend is not the first to experience something like this. Luckily, it only required a service to set things straight.
Whether watches are sealed or not, if left for an eternity ... doesn't really matter, they must be serviced. Period. Otherwise, I agree with your list of lessons learned and I am sure your friend has gained a bit of good judgement from his experience.
Honestly, I can't remember if this still is the case but it used to be the original strap accompanying the watch were signed Patek Philippe and replacement straps were marked with the interlocking letters PP. If so, it would be one more thing to look for with a sealed piece and possibly dodge a bullet.