Im hesitant because when I get hired - it is for the whole day, not just the formals. You want my vision? great - let me capture the whole day. To me this sounds craigslisty where you say I have a friend who knows photography, but we need you to just do formals and I am a photographer myself and wont step in *TOO* much. You can keep some photos and this is a great portfolio builder since the bride and groom are good looking and so is the Venue!! I see red flags all over the place. No thank you.
My opinion about hiring ( or most likely for free ) photojournalist friend - great, they know how to take photo journalistic photos, but a wedding photographer's experience goes much further than just knowing how to set up formal shots. There are so many little nuances that one probably does not know unless you are well versed in the wedding industry. What about directing cake cutting, getting ready shots, first dance, and just the bride and groom? What if they ask the photog for guidance? ( happens ALL the time ).
If this photographer friend is that good, then trust them for the whole day. This just sounds like someone on a budget who doesnt want to pony up and hire a pro for the entire day. What you dont get is that 3, 4, or 8 hours, I use my Saturday to shoot your wedding and therefore cant do another wedding - so I have to charge quite a bit *EVEN* if Im just shooting for 3 hours. Travel time is still the same, equipment expenses are still the same, car insurance, equipment insurance, using my Saturday up..... all still the same regardless if I do a 3 or 10 hour gig.
My best advice - You don't have to spend a fortune on a wedding photographer, but don't skimp out either. A good wedding photographer will want creative rights to using their images and legally you dont have the right to tell them what they can or cannot use UNLESS, you modify a contract, which you should be prepared to pay extra $$ for. A good photographer will want to capture the entire day and will charge you a decent amount *NOT* to, because they still have to turn away other potential clients for that date.
Honestly I kind of take this post as a slap in the face, be it the intention or not of the OP.