I can only think of two viable options that your friend may be referring to:
1. He’s going to mass German bombers, which is extremely flexible and depending on what the USA does, can be used to kill London or Moscow and perhaps take Egypt as a bonus as well. Your friend may be thinking to just open up with some standard play (except that building a lot of bombers is not very standard, but that aside), then wait and see what your allies are going to do and jump into your weak spot. Yes the axis can play an effective ‘waiting game’, as long as they keep expanding their economy.
2. He’s thinking G4J4. I better not go too much into the details, but if done properly, this is a very strong strategy for pursuing an ‘economic victory’. The typical path for the axis in this case is to crush Russia from both sides (Japan will ultimately take ~14IPCs/turn from Russia). This requires an extremely well balanced answer from the USA particularly. If they focus too much on Germany, Japan will get to ~100IPCs per turn during the late game and will eventually grab Hawaii -> game over. If, however, they focus on Japan too much, Germany and Italy can take Gibraltar (making it a fortress very hard to retake) and subsequently a lot of African/ME territories from the UK while Russia slowly collapses. Perhaps they even take Brazil… By the time the USA owns Japan, Germany should be able to take Moscow->game over.
Anyway, these are the two axis strategies I can think of and how their preferred road to victory looks like. It’s up to you as the allies to thwart this.
If you think your friend is a strong player, take his hints serious but like stated before, he may just be bluffing and not be getting anywhere. I hope you know your friend good enough to know what to expect ;-).