I think Obito will be like "Ok Naruto i will help you"
2 different things that may happen
1.Straight after Sasuke, Stabs him in the heart with an evil face saying he is not worth of being an Uchiha
2.After Obito helps, Sasuke quickly kills him and maybe reveals his true motives.
Kind of like what the villans do to people that turn good to bad like the Aizen and Gin thing.