My uncle left HK @ 1970 to pursuit a college education in US, and didn't go back to HK until last year, he has no problem getting back in to HK (BTW, he's holding a 1970's ID card, and they let him get a new smart ID without any problem) and he's living in HK with his wife now, his wife is a US born Korean American...i think maybe the problem that your friend encountered is because he's not born in HK, if he's born in HK then his citizenship is for life..