I bought a container of Wilton's white cake decorating icing and it sat in my car for several hours (unopened) in 90 degree weather. I had planned on going straight home when I bought it but got held up. Is it still safe to use or should I trash and purchase another one?

Trash it. That is too much heat.
 

AOpen it and see how it is. That gross wilton stuff hasn't got a natural ingredient in it , so I'm sure it'll be fine.;-)