my friend is dating someone 15yrs older that her and she is 16 is this legal & can he be arrested for this?

I would be more worried about her parents killing him,

But, there is no law on dating, there is law on having sex, the location of where they are at, here in GA, the age of consent is 18, so if they had sex, he would go to jail for about 10 years for rape.

But a 16 year old girl should not be dating a 31 year old man, he is a pedophile no matter what they say or think.

Check your state laws. They vary from state to state.