Go with your friend to sign the paperwork at the MEPS. Say nothing to anyone. Your sole job is to make sure the contract is not signed until there is a slip of signed paper in the contract packet that says something about Timmy going to the firefighter MOS school.
You will make him walk away if that paper is not there, and will cover his ears if they try some bullshit on him about transfering to it later.

That is what a real friend would do.

Spoken like a man who's felt the press of a red-white-and-blue wang at his bacon ring!

A lot of my father's friends at the plant were in, some prior to the end of the draft and some after. The ones after told him how important the signing for a MOS was, and he and my mother drove down to take the role of friend for me. The guys at the MEPS were trying the shit on me what my folks got there, I was so dumb back then I may have broken. These guys are pros, better than car salesmen.

PS: Because all my Dads Marine friends went to San Diego for boot I went there too.

(In Michigan they let you chose which boot camp if you are putting off boot for a few weeks).

Tell him to go ahead with the FF stuff, make sure he gets it in writing, blah blah blah, but tell him he doesn't want to be in a CE Squadron.
 
At some point in Basic training, a PJ(pararescue), an EOD guy, and a band guy will tell him a bit about volunteering for one of these gigs. Tell him to listen very carefully to the PJ, and take the PT test, perhaps go to indoc and/or farther. Tell him to be able to swim and run WELL, and climb a rope proficiently. Whatever the outcome, he will thank you, if he doesn't get through it, he should have FF to fall back on.