I didn't fully attend both? Graduation ceremonies don't always end with the presentation of diplomas. There are generally guest speakers and sometimes the valedictorian speech is after, not before (like for his graduation). I left about 30 minutes before it all ended and didn't attend the post graduation celebration that they had all planned (so, the wedding equivalent of leaving immediately after the kiss and skipping out on the reception). And I showed up 10 minutes before my sister's ceremony, it was a very tight squeeze. In our case, both my siblings understood the situation.

I'm not saying his answer is correct. It 100% depends on how important each of these people are to him. If my best friend I can't imagine living without told me their wedding was on the same day as a close work friend I had already RSVP'd at, I'd be canceling that RSVP. But if it's two people of equal importance to him, compromising might be the only thing he can do. Obviously, he should not RSVP for both receptions though so that one of them are not paying for food he's not going to eat or towards any drinks.