The point of a shotgun is to hit what you want to hit, whether it's a bespoke Purdey or an 870. If he hits with it and he likes it, then it's perfect for him. It'll kill just as well as said Purdey. Fact.
Also fact is that the nowdays largely discounted M12 has seldom been bettered in the quality of it's patterns. I personally much prefer well fitted high grade double guns, but I'll happily acknowledge the practicalities, and not everybody wants nor can afford the big ticket double guns.
I used to hunt with a guy who toted a H&H Royal. He would often get pissed at me and call me a game hog because I killed more birds with my lowly off-the-rack Merkel. He was just pissed because his vaunted H&H didn't kill any better, never conceding that maybe his gun didn't fit, or maybe he just wasn't a better shot.
Within reason it's not the gun, it's the shooter, and anybody who likes his 870 is welcome to hunt with me and if he outshoots me great, and he's shooting an eminently suitable gun.