I'm going to tell my friend he shouldn't have a cat if he intends to get it declawed
Have others told you to just keep your opinion to yourself?
I don't want to because I think its wrong
I think it's good to tell your friend that you think he shouldn't get a cat under those circumstances. Declawing is inhumane and irresponsible and you sharing you thoughts might persuade him in the correct direction.
But it's his cat, so I suppose he could do what he wants.
You're right that your friend will probably be able to do as he pleases with his own pet, though in some states declawing is already against the law.  However, you should still be honest with him about how inhumane the procedure is for the cat, and how it has lifelong detrimental effects on the pet's health.  Your friend might change his mind once he knows these things.