The half your age plus 7 rule holds up even then in my book. It's not like you turn 18 and have to suddenly dump your 17 year old partner.

19/2=9.5, 9.5+7 is 16.5. so if she's closer to 17 than just turned 16 than the rule holds up. He is on the lowest bar though.

You're going to find there's a lot of creepy guys who hang around your high school and try to get with high school girls because they're emotionally stunted and don't know how to attract someone of the same age. When you see a 22 year old guy prowling around a 16 or 17 year old, alarm bells should be going off. Your friend? It's nearing weird but if this is a one-off case I wouldn't be too surprised. It's like a senior dating a sophomore.