Come on Hal, when you are 18 seeing someone who is 22 could seem very weird. A lot can change in a person for those four years. No reason to think he might be "showing off". It's not like he went off about how nice her ass is or anything. Ease up
Back to the topic at hand, if you feel like this thing coudl develop then go for it. Your friend might have feelings for her as well, but if he has been shut down/doesn't have the guts to go for it then his loss. By all means, get to know this girl better if you like her.