My friend is 16 and she was starting to see someone who was 20, would this be legally wrong? Or just morally wrong?

Both.

Adults should not date minors, period. If they have sexual relations it can be considered statutory rape, a felony. Even if they do not have intercourse, it can be considered "contributing to the delinquency of a minor". People have been convicted of pedophilia for inappropriately touching minors. Some people think that as long as they don't have intercourse it's OK but it is not.

What's more, any mature 20 year old would know that the relationship is totally inappropriate and would not even consider a high school age kid to be a potential dating partner. A four year difference is nothing in your thirties, but at this stage in life it's huge.

Your friend should slow down - she'll grow up fast enough. She should date people her own age.

... People have been convicted of pedophilia for inappropriately touching minors. ...

Maybe, but I don't know of any state that has a crime entited pedophilia as such. Statutory rape depends on the age of consent in the particular state or country involved; it's often 16.

But the question relates to "starting to see someone". Dating, as opposed to sex, is not a crime.

If in the US it will depend on the state, in almost all, also the minors parents will have to give permission for the dating.