Remove the fender and make a measurement with the wheel back in place. From the center of the fork crown mounting hole to the center of the rim braking surface. Try to make a vertical measurement to determine the distance as closely to the nearest millimeter as possible. That will become the brake "reach" with which you'll specify the dual pivot caliper size.
Tektro makes both and internally nutted calipers. This has to do with the length of the central fixing bolt, and whether or not the threaded nut extends within the rear-most hole in the fork with just a flange or shoulder remaining on the outside, or whether the nut is entirely on the outside. Once you make the measurement, then we can talk about the two types and which you'll be able to use.
If you have your friend bring his drill bit set, then he can tell you the nearest size bit that fits within the fork crown hole. We'll need to know that, too. PG
EDIT: Vertical refers to the plane on which the front face of the fork crown lies. This may be slanted forward along with the line of the fork, or it may be in line with the head tube. Align your measure to the head tube, and you'll be accurate enough.