If he hadn’t stopped talking to the girl when he became interested in you again, he was most likely not getting what he needed out of her.
Thereby, whatever attention he was getting from her wasn’t giving him the emotional boost that he needed.
Given that he was showing you a lot of interest at the same time, this girl was most likely just a filler.
It isn’t surprising that she would be interested in him given his nature.
She may have truly believed that something more would come out of the interaction.
However, he probably always knew that she would not be enough of an emotional boost. He needed something more. He needed the kind of emotional boost that only you could provide when giving him attention.
If you believe that ignoring him will not encourage him to step up and ask you out, then you may have to consider stepping up and talking to him.
If your feelings for him are this strong and you already know that he is soon to graduate and you will probably never see him again, you may not have all that much to lose in taking a chance on talking to him again.
As long as you are okay with the strong likelihood that he would only want to engage in playful and flirtatious conversation with you and nothing more.
Thereby, it’s important that if you do choose to talk to him that you keep your expectations of romance with him tempered.
He told your mutual friend that he just wants to be friends and he still means it.
He may flirt with the idea of dating you from time to time, but so far, that impulse has never been strong enough to force him to act on it.