I have never used Reason so my advice is kind of general here, but if you find that FLStudio has a nice interface and sufficient tools to make your style of music, just stick with it. Despite what some people may think, FLStudio isn't some rinky-dink my-first-DAW software. It's very powerful and versatile and you can do a lot with it.
Why is your friend switching to Reason? What kind of music are both of you writing?