The Magpul forend may not work if his M500 has the forend riveted to the action bars. He would have to get a new forend tube (or whatever it's called) of the appropriate length and a length of MOE rail to go with it.
I used a CDM mount on my Maverick 88 that I got from SFRC but i see they and S&J Hardware are sold out. It worked very well for me but I gave it to my dad to help with nighttime skunk problems.