My two cents for what it's worth. If your buddy wants to get to a US major his best bet is to identify the quickest path and choose that one. Seniority is everything so any delay can hurt future earnings and quality of life. I know several senior FOs who make $270-330,000USD at US major airlines. As a captain you can obviously make much more.
That being said if his dream is to fly for EK then go for it. There is certainly more to life than just future earnings. The destinations, expat experience, cabin crew, equipment, and all around adventure at EK is hard to match anywhere. I don't see anything wrong with planning on spending 5 years at EK and then heading home to UAL, DAL, AAL, etc. It might slow him down a bit but he will make some good memories in the process. If he hates it a couple years of 777 or 380 time certainly won't hurt his CV.