A scientist would not generally make a claim like "elephants never forget" because it is essentially impossible to prove by observation, and is too general.  Forget what?, for example.

That being said, elephants have very large brains (as a percentage of total body weight)--among the most "brainy" animals in the wild kingdom, along with whales, dolphins and humans.  So it is likely that they can remember an awful lot of stuff, but probably not where they left their car keys.