If most of his travel is going to be to Europe or the US, then staying with DL isn't a bad idea, as he can get a lot of places in the US/Europe with one stop in ATL or AMS, There is also a couple time a week AF flight to CDG.

I don't personally know much about Peru based FF programs available, however if this move is temporary (couple of years or so), I'd stick with DL unless he finds himself needing to do a lot of flights within Latin America.