A co-worker has basically the same truck as me, only stock. I think he has 120-130 on the odo and was having a great ol' time driving it - misfiring/running rough, etc.

Turns out #2,4, and 8 (if I recall correctly) were bad. He had them replaced by his mechanic - should I tell him to do the rest too? Or should he wait until the rest go bad? Basically, how long do/can they last? He honestly doesn't put more than 3-4k on this truck anymore as he drives a few other vehicles as DDs.

Lastly, is it common for them to have hairline cracks? Thanks in advance

 

I would replace all COPs and plugs at the same time and you should see a noticeable difference.
COPs have been known to go out after 60k and the cracks come from heat and age.

 

Coils convert low voltage signals from the PCM ~ to pulse. Overtime, the copper windings break down/shear and become weak. If their not misfiring, they could very well be weak enough to cause the engine to run like it would with a worn out set of plugs.