I was playing with my beother pokemon cards and he had
a rocket charmander, and he evolved it into a base
charmeleon. I told him he couldn't do that.
My question is if hes using a rocket charmander. Can
he evole it into a base Charmeleon?
Am i right or wrong.

Charmander can evolve into Charmeleon or Dark Charmeleon.  It does not matter what sets the Charmander or Charmeleons are from as long as the official name of the two cards are "Charmander" and "Charmeleon".  What your brother did was legal.

Scott