I would concur with staying with United. If he had trouble getting an award on United he's in for a rude surprise with any other airline. US has no online award system at all for *A awards and no one-way awards. AC fuel surcharges make them almost unusable. Etc.

If he was only interested in status and not awards he might check whether it's worth shipping 18k miles to Aegean for the status and then keeping the rest on United for the awards. But I think that's not such a great trade myself. It would mean getting domestic lounge access but it doesn't sound like he flies domestically much anyways.

If he's just barely doing 50k on United he might miss *G. But if he's doing 70k+ I wouldn't expect him to have much trouble. He does have to be careful to ticket his EVA flights on United tickets. That should be possible by calling in to United reservations and having them issue the ticket or by going to a travel agency and explaining why it's important to do so. Doing so should allow his EVA flights to count towards the minimum spend and in theory it shouldn't change the price but ymmv.

Alternatively he could change his address with United to an address in Taiwan or some other overseas location he can use. A business address, friend's address, or post office box for example. It would mean missing out on promotions that are only available for US residents but it would exempt him from the minimum spend.