A lot of girls are ADD to the max. You can have their complete attention and think things are going well, and then they catch a glimpse of something "shinier and prettier". Then they will go for that. It's rooted in their biology. It doesn't have to be a guy. It can be the opportunity for more fun. Girls act on instinct and emotions. That is why learning when to pull the trigger is golden. For me, it took pulling the trigger too early and too late a number of times to finally get my calibration right, and it's not even right half the time. Just go for the gold every time. If you have enough game, the girls will let you make mistakes over and over, and eventually you'll fine-tune your sense in when to escalate.
To your specific example, I would agree with Roosh that isolation and booze work best. I would go balls-to the wall to close her.
Next time that happens, you could have invited the dude to come and hang out at your friends place. If he refuses, you can call him out on trying to steal your girl and cockblock him, but in a subtle way. Say something like "I understand, you are such a nice guy for driving her home." then look at her and say "You should really give him a chance. He'd probably make a perfect boyfriend. He's super nice". Platonic or not, he was trying to fuck her that night or at least cockblock you. Make him pay.
To your specific example, I would agree with Roosh that isolation and booze work best. I would go balls-to the wall to close her.
Next time that happens, you could have invited the dude to come and hang out at your friends place. If he refuses, you can call him out on trying to steal your girl and cockblock him, but in a subtle way. Say something like "I understand, you are such a nice guy for driving her home." then look at her and say "You should really give him a chance. He'd probably make a perfect boyfriend. He's super nice". Platonic or not, he was trying to fuck her that night or at least cockblock you. Make him pay.