I have read somewhere (sorry I lost the reference) that credsticks were not hackable because informations were not stored in bits or pulses or whatever but into micro-plotholes and thus was beyond the reach of any player

This is the explanations I use in my games, having the luck to have only players with a sense of humor (and no real decker)
But BitBasher has a good realistic explanation for this I think. In order to hack a credstick, you have to hack the bank. I don't know for sure if Zurich orbital has the monopole for credstick transaction in

but I believe it is the case. Corporate Download is pretty clear on that point : decking into ZO is the hardest and most dangerous task for a decker.
But if your player wants to forge some credsticks, it may be a little bit easier to deck into a megacorp financial system, as many if not all megacorps use an internal money for use in their arcologies and internal transactions. Such money may be convertible in nuyens in special conditions. It may be easier to hack than ZO but it is still a challenge even for world-class deckers.