Being a web developer, I hear this type of question all the time "how long will it take me to learn everything and master this language" This is basically the same as this question you are asking and I will answer it with the same answer I provide to them.
You do not have enough years left alive to master it, when you are asking this question, what do you consider to be "mastering" poker? You are never going to reach a point where you know everything, it just isn't possible. Look at the poker pros that have been playing for a huge part of their life, won millions and millions, but still they have a coach? Because there is still always some part of your game that can be improved.
Now if your friend means the basics, like hand strengths, how antes and blinds work and the different types of
poker games/formats. Then this information can be learnt within a day for sure, within a few months of studying, he could reach the stage of being a winning player if he takes it serious and puts in the work.
You can learn as much as you can for as long as you can, but there will always be something to learn tomorrow.