What did John do when Lenina arrived at the light house?

You can find the answer to this towards the end of Chapter
18.  We are not explicitly told that they girl who comes down out of the helicopter is
Lenina, but it is certainly implied.

What John does when he
sees the girl is to attack her with the whip.  He starts calling her various words that
all mean something like "whore" and he starts whipping her.  He is doing this because he
blames her for tempting him.

After a while, his whipping
her turns into an orgy that he takes part in.  This is why he kills
himself.