Auntie advice to niece

So my niece who is 13, came to me yesterday about relationship advice. She has recently been talking to a guy that is 18 and they really are feeling each other according to her. She can only come to me about this because her mom would kill her if she finds out that she’s dating and I’m her favorite. I was shocked and couldn’t say anything because I never knew 18 year olds were into 13 year olds. I told her I would give her an answer at the Thanksgiving get together. Should they continue dating? Why or why not. Just help me out here please.

Update: the guy is currently done with high school, he buys her a lot of gift and call them “spoiling” her. I talked to him before and say he really love her and all that bs she’s falling for. She came to my current boyfriend and he told her No but she thinks he saying that only to be mean, because my boyfriend always jokes around like that.