A friend is about to commission. His goal is to eventually join the Special Forces. What branch should he pursue? CA, MI, SC?

Have him go to ranger school.

SF Officer Candidates need more than small unit tactics. Strong knowledge of small unit tactics will help them in Selection and in the Small Unit Tactics phase of the Special Forces Qualification Course, but is not the only key to success. All too often people get fixated on the fact that SF detachments only consist of 12 men. However, the primary reason why SF detachments exist is to be able to conduct unconventional warfare or interface with foreign military allies. 18Bs and 18Cs are expected to perform as/or advised Company Commanders. 18A Detachment Commanders (O-3s) are expected to perform as/or advised battalion Commanders. SF soldiers are expected to be able to plan and write operations orders from the Division level all the way down to the squad level. So the most successful officers don't just know or focus on small unit tactics. They know and understand the doctrine of combined warfare in a Joint, Interagency, Intergovermental, and Multinational environment. The best 18A Candidates are those who did successful PL time, and then served as an XO or on a battalion staff because they need to know and understand higher level operations at the company and battalion level.

...and if he pursues that route he'll miss his window to apply for SF (if active duty).