My buddy and I are going to be brewing tomorrow and the 5 lbs. pilsner extract he picked up a couple of weeks ago has about four inches of foam on the top of it. Is the extract okay? or is something already starting to ferment in it?
Yes, the extract has begun to ferment which can happen if the extract is not refrigerated or if you don't treat the surface area to prevent this from happening. I believe it's able to do this because of wild yeast and any residual humidity it may have come in contact with. You can use the extract, but your OG will be lower due to the sugars that have already been consumed by the yeast. The wild yeast will be neutralized when you boil the wort.
John Arthur, who ran the homebrew store in Boulder years ago used to spray vodka on the surface of the extract in every 5 gallon bucket of extract that he put on his heat rail for dispensing. The buckets on the heat rail had their lids loosely placed on top for dispensing, and he used the vodka to protect the extract so it wouldn't ferment. I do that to extract I buy that I'm not able to use right away.
BTW, I made a very nice 2.9% ABV stout using extract that had begun fermenting before I got around to brewing. I enjoyed it.