There is actually a strong relationship between information and guessing.
For every message, we can talk about the data it contains and the information it contains. Both are measured in bits, but they're not quite the same thing.
The data in a message is just its size (eg. how many times we flashed our light, or switched our electricity, or whatever we did to transmit the message).
The information in a message is how much new knowledge it gives us, or how much it improves our guesses about the world.
Information and data aren't the same thing. If your friend can reliably guess the message you'll send, then you're not actually sending him any information! You could send him an informationless message faster than light without breaking causality, since it won't change anything that happens (he already knew what it says).
Information is precisely how unpredictable your message is.
In terms of causality, transmitting information is essential. If your friend can guess what to do, even without receiving your message, then you've not caused his actions at all! They were pre-determined, perhaps by some prior meeting you both had before zooming away from each other in space ships.
In order to cause his actions, you must be able to affect them; to alter the probability that he does one thing or another. That change in probability is directly related to information: if you're equally-likely to have him do one of two actions, then telling him which one to do takes 1 bit of information. When there are many actions to choose from, with varying probabilities of being chosen, then more information is required. It's possible to have "fractions of a bit", eg. a choice between 3 equally-likely actions takes 1.5 bits, but only whole numbers of bits can be transmitted, so we must round up.
The better your friend can guess what you're going to ask for, the less information he obtains when you tell him. If your two examples (sending a message vs. him guessing) were actually the same, then your message would contain no information, and hence couldn't violate causality.