I didn't follow this story that closely at the time, so I could be mixing up some details.
I believe that Nick was always intended to do a bit of stand-up as an opener at a comedy show before Chrissie came on as the headliner. He talked about it on his nightly shows—first as an idea like, "hey, wouldn't this be fun to actually do this?" and then more later once Chrissie agreed and the details were made official. He was talking about it months ahead of the show.
But then
Chrissie Mayr had to pull out of the show because her dad was killed. So then Nick was left to "fill in" for her in the sense that there was no longer a headliner—but a bunch of people had already bought tickets, so he had to try to stretch his "internet personality" schtick into a bigger show.
Presumably, Riley came out primarily to see and film Nick's part of the show, which was originally intended to be just a small opening act.
I think Riley was going to hang out with Nick or do other stuff during the trip out there, but the fact that Nick was going to do an opening bit was the main draw that got him out there and made the whole occasion happen.