Candid photos has less to do with how far you are situated with a long telephoto and more to do with "how" you shoot. During the wedding reception, you will also be using a flash which is going to bring attention to you as soon as the first photo is snapped. You'll want to keep the camera at your side until you see your next shot. You'll want to move around a lot. You have to be quick.
Unless outdoors and posing your subjects, shooting indoors in a smallish closed in area is going to be less than ideal with "most" super telephotos. People get in the way, lighting is low, flash is less effective, obtaining a fast enough shutter when handholding are all going to be issues. Add to that the 100-400L (albeit a wonderful zoom) is not the ideal glass indoors. Yeh.. there will be times that a telephoto will be required for weddings but in those cases a fast medium telephoto like the 70-200 f2.8L is going to shine.
Since you are doing this for little pay, as a favor, and on a budget, I would recommend focusing your attention on getting the normal regular shots of the wedding rather than get all creative with wide-angles and telephotos. You have a lot on your plate right now... in this case, I would focus on the basic sets.
* Bride just before the wedding
* wedding party walking down the isle
* Groom
* Bride and father
* exchange of vows
* exchange of rings
* Bride and family
* Groom and family
* Bride, groom, best man, maid of honor
* Bride maid of honor
* Groom and best man.
* Bride and groom + their immediate family members
* Grandparents and such
* The cakes
* Wedding party entrance into the reception hall
* First dance
* Bride and father
* Groom and mother
* Cutting of the cake
* "Dollar" dance
* throwing of the bouquet.
etc.. (keep a cheat sheet in your pocket)
Once you have those done... then the candid stuff is icing on the cake.
btw... you should scout out the locations prior (if possible) to get a feel where you need to be for the best vantage point... going to be very important as you have to be quick with composition. As suggested, take some test photos to get a feeling on how you will have to work. Talk to the church to get a feel on limitations that you will have to work around... How close can you be? Is flash allowed? I assisted on one wedding when nothing was allowed... not even cameras. It was a religious occasion that should be kept strictly that way. Fortunately, they allowed for "re-enactments" after the fact... This is information that was extremely important to know ahead of time.