We have a friend who is registered lost since 5 weeks. After the police interrogated her husband, he admitted that he launched her body in the sea, but till now (today) we did not find the body.
My question is as follow: What kind of pray we have to do for her is it GAIIB pray, or we have to wait till the body is found and we do Salat Janaza?

All Perfect Praise be to Allaah, the Lord of the Worlds; and may He exalt the mention of Prophet Muhammad as well as all his Family and Companions. 

Basically the funeral prayer should be offered on a dead person whilst placing his corpse in front of those who pray on him.  As long as the search of the corpse is going on, and getting it is possible, it is better to wait until you find it.

However, if it is strongly believed that your friend has died and you cannot find her corpse, then there is no harm – Allaah willing – to perform the absentee funeral prayer on her.

Allaah knows best.