According to the Star website does not have to be on the same flight.
Lounge Access
International First Class
Customers who present an International First Class boarding pass, regardless of the Star Alliance carrier they fly, have lounge access. They, and one guest, are welcome to visit first class lounges or any other members' departure lounges when departing on a Star Alliance flight. Note: AC, BD, OS and SK do not have international first class service.
International Business Class
Customers who present an international Business Class boarding pass, regardless of the Star Alliance carrier they fly, have access to all members' designated business departure lounges. Guests are not permitted.
Star Alliance Gold Card Holder
Star Alliance Gold card holders and one guest, have access to any lounge that is identified with a Star Alliance Gold lounge sign when departing on a Star Alliance flight. Note: United Airlines' Premier Executive and Premier Executive 1K customers and US Airways' Chairman Preferred and Gold Preferred customers have access to U.S. domestic United Red Carpet Club and US Airways Club locations only when they present a ticket or boarding pass for a same-day international flight.
Eligible Guests
Any guest of an eligible customer (International First, Star Alliance Gold card holders, and eligible paid member) must enter the designated lounge with the eligible customer. The guest must also depart on a Star Alliance carrier, but may travel on a different flight.