They have all sorts of issues with Einstein, and most claim gravity is either differences in density, or something to do with di-electricity and magnetism. Just saying, I've been down these roads, and they have answers. The guy I know isn't some crazy person, and he learned all the same facts we did. It would be worth your while to actually look into what they believe. Like I said, my friend, and others he's shown me on different platforms, have done way WAY more research into this than I have ever done into what I was taught to believe about the cosmos. It's not easy to prove them wrong. Also, something he shows me all the time, is issues with the curve of the Earth. Neil DeGrasse Tyson states that you cannot see the curve of the Earth. Other people, like Bill Nye (not an actual scientist) will claim that you can see curvature from an airplane. It's really weird, man.