There's a couple things at play here and some context that needs clarifying.
Did the two already know each other/were they part of the same crew? If so, it still isn't cool to pressure someone like that, but it's less weird to be invested in seeing someone you know accomplish something. If a stranger came up and gave me the ol "I'll do it if you do it," I'd be weirded out. That being said, I've definitely entered into that agreement with a good friend and I did feel like trying the trick alongside him not only helped me land it, but made the process much more enjoyable.
That brings us to another factor to consider: Was the trick something that could have resulted in serious injury? Way too gnarly in general, or just above the kids skill level/comfort zone? Recently a younger friend was on the cusp on learning rock to fakies. Admittedly, i dogged on him a bit to keep trying it because I knew he could do it, he really wanted it, and he'd safely bailed a million times. I still felt a little goofy about it, but I think that's completely different than egging someone on to do something too challenging or dangerous. Commitment is personal at the end of the day, and it's really only cool to help "hold the homie to it" if they're actually ready to try it. Even if someone
could do something, if they're not down to try it, then that should be end of story. Especially if it's risky.
And finally, my last question: Why is the older bmxer from the story posting here?
Oh, y'all really are that fried.
Throwing a fit when the kid doesn't try the trick in this situation is inexcusable and embarrassing. Even if the two were the same age, it'd be cringe. Respect to the kid who stood up for himself.
Edit: it's okay to egg someone on if they're bunting. Like, say they claim they can nollie 360 flip over a spot that was recently featured in major videos/photos. Your friend is talking about a trick on par or more difficult than one done by a big name pro at the same spot that got significant coverage. He's so sure he can do it, he starts crowdsourcing funds to fly out to the spot and make it happen. There's a buzz online about the claim, and skate media platforms cover the buildup to his attempt only for him to suddenly disappear. In that situation, he should be pressured to actually try the trick by any means necessary.