Be aware
Does your friend in the military have legal residence in FL? You do (I assume), but if he doesn't, it may be considered a sale to an out of state resident. When I was in the military (decades ago), my legal residence was my home state address of record (no matter where I was or how long I had been there).
This may have changed since then, so you ought to check it out. Since your friend is 19, I will assume you are not talking about selling a handgun. If you are both legal residents of the same state, a face to face private sale of a long gun is generally allowed (but check your local laws). If you are not legal residents of the same state, you must go through an FFL, and possibly two FFLs, depending on the exact circumstances.
If a private sale is allowed, a written bill of sale is all both of you need (provided he meets the legal requirements for purchase/ownership), in order to be ok.
Check with the cops, but also check with an FFL. If there is any question at all, use the FFL (pay his fee) to handle the transfer. That way all will be done in accordance with the laws.