A BJ doesn't have to end up in her mouth.. oral can be used as just tease/foreplay... or when she's doing it.. give her a heads up when you are about to cum and she can finish you off w/ her hand.
If this is the decided way.... PLEASE don't slip up and tell her too late
this is no fun especially when you are NOT expecting it! lol
If so.. she might never go down there again!
It does take some getting used to.. and after a few times like this.. she might be more open to waiting a little longer before you cum.