In a case such as you outline, your friend is making a legal argument, not a moral argument. I happen to believe he is wrong about the legal argument—and from a legal, not a moral, perspective—but Catholics are free to hold differing opinions on points of human law, so long as they do not dispute the moral law promulgated by the Church. Since your friend evidently does not dispute the moral law and is working toward a way to fix human law on an issue of grave moral import, then he is not acting contrary to his Catholic beliefs.
That said, for your friend to declare to other Catholics that he thinks Roe v. Wade was correctly decided is scandalous (i.e., it can lead Catholics to believe that a moral evil, in this case abortion, is not evil). He should be extremely discreet about his opinion on this matter, mentioning it only in private to those few individuals he knows are formed well enough in their faith, and know him well enough, not to be scandalized.
