Maybe I'm just lucky and getting grouped with the right people because 99% of the time, I could swear this isn't even the issue.
Either way, assuming the battle isn't too hectic, why not just speak up as soon as you see this happen the first time? "Hey, just a heads up, you can dispell X! Keep an eye out for it!" or something...or anything along those lines...
It's better than saying nothing and if they listen, you've just helped them improve and insured a better experience for their future party members. #grouphug