What aircraft type would he fly for Korean Air? Obviously on a narrow-body fleet like the 737 he would be flying primarily regional flights. On larger aircraft (A330, 777) he might be doing more long-haul flying with the associated jet lag, etc. but he would visit some interesting places. I am not sure about the UK base but clearly that would entail a widebody aircraft like the 777 or 744. Living in Korea would be a culture shock and it could be expensive.
With Ryanair you get point-to-point flying and no layovers. But, you get to sleep in your own bed every night and you get some predictable time off with schedules. You fly one aircraft type but you fly mulitple sectors per day. Some people prefer that and want to focus on European flying.
What does your friend want in the long-term? It might be ok to start at Korean Air (if he has the option) to try that type of flying and then return to the UK and Ryanair with more flight experience if long-haul flying doesn't appeal. Widebody experience might also make him more appealing to BA or Virgin if that is of interest in the long run. That's my 0.02.