5FT Step?

[Hypersmurf]

Wizard takes 5 and his action stops, as Billy's trigger goes off. Billy also takes a 5 foot step, gets his full attack and, since Mr Wizard is in reach of Billy, provokes a AoO when he starts to cast. (Ouch!).

No, he gets a single attack. A full attack action is a full round action, not a partial.

PS Should Trevor get smart and NOT take a 5, Billy gets a AoO and then can abort his ready action, for 1 more attack and now his initative is AFTER Willy.

No, you can't "abort" a Readied action. If your trigger doesn't come up, you take no action until your initiative count comes up in the next round.

-Hyp.