actually, dogs and cats have about the same intelligence. But cats appear dumber cause they are 'arrogant' in their nature.
dogs are loyal and will do their best for their companions. while cats just lay around and don't do anything, they could learn but they are just too lazy compared to dogs
sooo dogs FTW
and your dog lover may have been referring to a dog's want to do its best as being dumb, i call it hard work/loyalty to its master
edit: yeah i should leave this thread before all of you cat lovers try to kill me