I've called a judge over on several of my own friends! Like Pop said, tell them "you're taking a bit long, dude." and if it doesn't get any better raise your hand and ask a judge to watch the pace of play. And since I wasn't ever a jerk, we're still friends.
That's another negative of slow play. Calling over a judge, explaining your situation, and waiting for a resolution all takes time. This is exactly what the slow player wants, more time eaten off the clock so he can maintain his prize advantage at the end of the game.
This has always been a problem in pokemon though, so there's not a whole lot that can be done. Gamesmanship through slow play or "slower play" like eating "a lot" of time through multiple shuffles, extended searches, taking too long with notes on time walk, are all things that fall under the hard to call guideline.
Its just like with basketball and charges/blocking, and football with pass interference (or I guess illegal hits now). These are some of the harder, if not hardest rule infractions for a ref to cite cause there rule book is wishy washy or there's not firm technology, evidence, time in place to make this split decision call.
When my best guidelines are "a lively game pace," its ultimately up to the head judge at the event to determine whether or not my opponent is playing too slow or not or if he's taking too long on time walk. Punishment is also different, some judges might just give a warning, some might give a prize penalty, or even a game loss! All of this I have to take into consideration when deciding to "waste my time" or not by calling the judge over against a slow player.
PS., I would love chess clocks to be instituted in pokemon and maybe only give each player a net 15 mins to complete their game (summing up to 30) but this will probably never happen so slow player's who abuse the system (the ones who are doing it right) will probably continue to do well in our system and I'm left trying to attempt to "outplay the slow player," which isn't too hard I guess cause a lot of the slow players (the ones who aren't doing it right) just end up beating themselves by not playing fast enough.