Beeayeate's picture is spot on for the early sixties, it matters not that the picture is of a T4, T3's were in the same scheme. The picture does not convey the exact shade of fluorescent dayglo red/orange, however. But change the code letters to 21, and everything else would be right, down to the small yellow band on the fin.
I stand to be corrected, but as far as I know the JP3's never wore yellow; for certain the vast majority were delivered with dayglo - the markings of the old Airfix Jet Provost kit , showing it with yellow bands, may have perpetuated the yellow bands myth.
Paint dayglo gave way to dayglo strips, not such an attractive colour scheme, in the early to mid sixties. The first JP in the red and whire colours appeared in 1968, in the following years those that were not retired were repainted.
As for getting the colour scheme exactly right, I'd suggest looking at aircraft modelling sources, things like Decal sheets sold by Hannants .
If your friend does choose the dayglo scheme, he will have made a a very rare and eye-catching choice, I can't think of anything flying in those colours, not in the UK anyway.