Zeable on Family Tree Circles

I have a friend whose surname is Downey. He thinks he is descended from Joseph Downey who was a seaman on the Lady Penryn in the First Fleet. Joseph had a male child with Sarah Bellamy about the time they arrived but the child known as Joseph Downey Jnr. died very shortly after he was born. There seem to be no other entries for Joseph Downey so I suspect he went back to England on the ship as seaman was his vocation.So in my opinion my friend would be descended from another Downey somewhere. Does anybody have any ideas about this family or any single person either. ?