Why don't my [23/M] friends [21F, 22M] want to get along and is there any point trying to reconcile them?

During October 2014 me and my mate Jimmy both broke up with our ex's and fell out with each other. Jimmy wanted me to unfollow his ex, Becca on social media and stuff otherwise he'd start tweeting nasty things to my ex so I told him to fuck off, because I didn't like his manipulative behaviour (he wasn't a nice person but that's another story).

We fell out and did not speak for about 3 months in total and in that time I became mates with his ex. We never hung out or anything just chatted on WhatsApp etc.

Anyway around Christmas 2014 me and jimmy make up and become friends again but I'm still friends with Becca because we have a good rapport.

In April 2015 he stops talking to me because he is upset that I'm still friends with his ex (never confronting me about it beyond the initial fall out in October). I stop talking to her for about a month and try to reach out to him to mend the friendship because I hate drama. He doesn't respond, deletes me off all social media and I call it a day, a lost cause. At some point between April 2015 and now he's blocked me on all social media too.

Basically a lost cause so now I am close friends with Becca. We hang out now, and support each other well as friends. She's a great girl.

Jimmy and I had a mutual friend who I'm very close to called Robert [22/M] and we remain very good friends. Becca and I are going to a festival tomorrow, Robert is also coming but i was under the impression he was arranging his travel himself (he is) so Becca is driving her and I and her boyfriend up there whilst I meet up with some other friends for the whole day.

Robert messages me today asking how I'm getting up there and if I'm getting driven, could he grab a lift too. I say yes I am and say "Im travelling with Becca I'll ask if it's okay for you to get a lift with her" and she says "Thanks for the offer but I'd rather not have to make conversation with her in the car for an hour"

So fuck knows how he's getting there tomorrow probably getting a train but it's pure effort for him, even if he doesn't get on with her or make it uncomfortable I'd just grin and bear it personally

This annoys me because we are 20-23 year old adults and drama from over a year ago is still affecting our life and i really thought he'd not be bothered about getting in the car with us for an hour. It's only a short journey and he's generally someone above the personal feuds of others so in this case it shocks me really.

Is there any way I can get Robert not to hate Becca in the future? It's very inconvinient having to keep these social circles entirely separate. Like by spending time with her I'm having an illicit affair or something.

tl;dr: Drama caused falling out over friends ex a year ago. I am friends with said ex. Other friend won't associate with ex at all. Any way to stop these immature politics, or should I just leave it be?