I don't think there are official rules. If I were gamemastering, I'd say that he was pushed back two squares (15') and you have the option to follow him one square. Players are usually happier when rounding goes in their favor, so few will say "he should only go back one square."
If the target can only move diagonally, the target would only move one square. Since this is unlikely, I suspect the target would move one square diagonally and then one square either up or to the side. I'd say the player would get to choose which direction.
So if you have columns ABCD and rows 1234. Target is at C3 and player hits them at D4. The creature moves five feet diagonally to B2 and then either goes to A2 or B1 since that would be 10 feet. A1 would be 15 feet and thus too far.
Darn, I was hoping there was a movement chart somewhere that answered this scenario.
With that said, I think you guys have covered most of the possible answers. I am not sure which answer a player would like the most. The move one diagonally and one up/sideways gives alot of control to the player. The two moves diagonally gives the player more bang for his buck with a bull rush. Of course, moving one square diagonally would make a player feel cheated.
I wonder if giving the player a 50/50 chance of making the target go 5ft or 15ft diagonally is another workable solution.
It would. But I think the important question at this point is "which solution is the most fun?"