This question reminds me of a scene from the original Star Trek episode The Corbomite Maneuver where the Enterprise is hopelessly outmatched and the logical Spock makes a chess reference indicating they have lost. Kirk replies to the effect that instead of playing chess they should play poker, and proceeds to make a bluff that succeeds in getting them out of the situation. Of course later we learn it was less the fact that Kirk bluffed, and more that the aliens hadn't really intended to destroy the Enterprise. In Star Trek the Next Generation we see a similar pattern, where second in command Commander Riker is great at poker, usually managing to somehow induce the other player(s) to make the wrong assessment as to whether or not he is bluffing.
Tell your friend to forget about all these media representations of the game!
The basic principle of the game is known as "ABC Poker". There are plenty of guides out there that Google will find for you that go deeper into exactly how to play, but the gist of it is that poker is a very well mathematically defined game, and good play involves making correct decisions based on the statistical outcome of play. Nothing fancy, just solid play. The classic example is that of drawing to a flush - if in a particular hand you have a 1 in 4 chance of making your flush with the remaining card to be dealt, and that would give you the best hand, then you should not continue unless you will be getting the proper odds (direct and implied). Correct decisions have positive "expected value". Part of becoming better at poker involves learning how to identify these situations properly. For instance, a beginner might see they are getting proper odds to make their flush, but ignore the fact that their opponent likely could beat a flush.
Another important principle involves what "level" of poker you are playing at. At level one you know what your hands is. At level two you start thinking about what hand your opponent has, trying to put them on a range of hands based on their play and adjusting your play accordingly. You will need to identify how the other player is playing at the moment (e.g. loose-aggressive, passive-tight, etc.) in order to try to put them on a hand. At level three you will start thinking about what hand your opponent thinks you have. At this stage you will start to realize that certain lines of play will work because they play off what your opponent thinks you have instead of what you really have, and conversely you will abandon certain lines of play, realizing they aren't believable based on what your opponent thinks you have. For instance, certain spots will become optimal to bluff in because your opponent believes you have the cards that allow you to, and other spots will becoming obvious bad spots to bluff. Higher levels involve asking questions like "What does my opponent think I think she has", "What does my opponent think that I think he thinks I have?" etc. This is very advanced play and requires that you first of all know whether it is even appropriate to be thinking like this - in fact, for optimal play you should generally play one level higher than your opponent, especially at higher levels.