In that particular situation, I'd let them carry on doing what they are doing.
They've been playing an apparently good game, but when you've spoken to them they apparently don't know the first thing about poker. I may be cynical, but I see that there is one of 2 situations here.
1) They really don't know anything, and as such, any advice/encouragement (purposefully good or bad, either way) you give them will come across as you telling them that they are an idiot and have only succeeded in being very lucky, and that's unnecessary.
2) They actually DO know a lot about poker, and are instead trying to hide that fact, or are even trying to get you to open up about how much you know and get a read on your ability and so forth. If this is the case you REALLY don't want to volunteer any information as that could be what they need to start eating into your stack.
Either option, I see it as a bad idea to start down the 'advice' route and instead you're better off at most, just commenting that they appear to be having a good game and wish them luck.