How far could you position your camera from the tree? I suggest mounting the camera on a tripod, use a cable to trip the shutter. The reason: maybe they will love this photograph and order a large print from you. I have several 30" x 40" prints on display in my studio and they can provide income to get extra film toys!
The reason I ask is perhaps you could get away far enough where the tree would be in the background and the folks in the wedding party would be in the foreground (have them stand away from the tree and fairly close to your camera - perspective!). If you could get far enough away then maybe you could capture the entire tree. A wide angle lens could help but I recommend checking things out as you don't want to distort the bodies of people in your photograph.
What time of day will this take place?
At any rate, I think this is a good idea, a great idea if your clients wants this as part of their wedding day photographs.