Happy to recommend!

For Reynolds, if you want to try a standalone novel I would say House of Suns is a great place to start. Very fun read. If you want to get in to the meat and potatoes of his writing instead, I'd say just go ahead and jump right in with Revelation Space, which is the first full installment of the series by the same name.

As for Greg Egan, I started with Diaspora and after reading four or five others I think that was a lucky start. He's basically the farthest point on the hard side of the soft-hard sf spectrum, and if you find Diaspora to be not quite to your liking, you almost certainly won't like his other works as they are even more whacky. The other startinf point that could be good is Permutation City which is SLIGHTLY more grounded in the modern day and not as far-reaching in some ways. Still, it's a crazy book because he doesn't know how to not write crazy books. He always has resources on his website to help parse some of the more mathematical concepts so perusing that can help, or make things worse lol.

Addendum: my friend is an avid reader like me and also didnt care for Red Rising, largely due to the pacing, so don't feel bad if that isn't your cuppa tea.