First they sign up for Aegean Airlines' FFP. Then, they book with any Star Alliance member (Lufthansa presumably in this case). They give Lufthansa their Aegean Airlines mileage number.

Then, after earning 20,000 miles within a year (1 round trip if they buy a 100% mileage earning fare, or 2 round trips if they buy 50% earning fares), they get *G status, which gives them 3 free bags, lounge access, first class checkin, etc.

They just have to make sure that the fare they book is mileage earning before confirming their reservation... Most of the cheap fares will earn 50% miles, but that's still enough to get *G with 2 trips.