i am probably too late to be of any help but your friend should be scoped right away - not that it is a dire emergency per se, but to hopefully avoid one.
he should see a gastroenterologist (commonly refered as "GI") or a surgeon who performs an upper endoscopy (aka "EGD") to try to get the bottle cap out while it's still in the stomach. the quickest way to do that, unfortunately, may be to go to the nearest ER. they can even do a quick x ray to determine where the cap is at.
the cap in the stomach is not an immediate problem per se - it will not hurt the stomach (assuming that it has passed through the esophagus). but the problem may be when it exits the stomach into the intestines, the cap may cause a tear in the bowel causing a perforation which would be a surgical emergency. but my guess is, the cap will stay in the stomach unaltered for a hours - and then it should be removed with the endoscope.