I got a similar offer. In my letter, it said the enrooled friend must complete a qualified international roundtrip (in Y class, business or First) between May 1 and August 31. We will both earn 7000 bonus miles for Y class, 10000 bonus miles for Business class and 15000 bonus miles for First class.

Again if the enrolled friend fly two or more international roundtrip, he will qualify for Premier status. I also have 4 of these referral, email me if you are interested.