Yes, it's not normal.  Speaking from experience.  A guy that we all knew had a rotating array of younger girls.  Never anyone younger than 18 but still.  He had an apartment, always had beer on hand, and partied a lot. When you a first-year college kid with college kid friends, it's cool.  I was never attracted to him but he hit on me, asked me to go places with him. 

He was immature and sad.  He could have dated women his age (like 30) but he was a bit fucked up.  Maybe your friend is immature and fucked up.  The girl doesn't know what she's doing because he probably has a job, a car, maybe his own place so that's cool. 

Some women have the older guy fantasy and seek them out.  It's all kinda creepy.