I have this friend who is 16 yrs old.. and he likes this girl who is 18 yrs old.. she likes him back.. but the guy who is 16 doesn't know if it is wrong.. is it wrong for both of them to go out?

Nothing wrong with him liking her, if you want to know if having sex is illegal that is another question.

But if she does not mind dating a younger person, there is no issue in dating

There is nothing wrong with a 18 yr old and a 16 yr dating... as long as they don't decide to have sex... that would be a different story and its called "Statutory Rape"...
I wish them the Best of LucK