I have heard that the speed of chemical reactions doubles for every 10 degrees C.
If you accept that to be true, you can assume that the beer will basically just age at a much more extreme rate. Bottles would be a bigger issue, because the swinging temperatures would promote more of a gas exchange through the crimped cap. In a keg, this may be a lesser issue.
So in a nut shell, storing at 90F is not a great practice, but I guess you do not have another option. Drink it up as soon as you can.