sorry cant help with size but should be your J dimension for the rig. get a quote for one from selden or similar manufacture they should be able to tell you.
Thanks. I was also looking for dia. of tube as an option is to buy the components and make one up although the original had tapered ends so I don't know if this is readily available.
The tapered ends enable you to have a larger diameter pole which is therefore stronger for its weight. Strength can be important if you sail shy with the pole pressing on the forestay.
I suspect that your friend would be happy with a constant diameter pole so the pole diameter is dictated by the fittings you buy.
I assume you gybe by end for ending the pole. (not dip gybing)
So if it is a wide large spin then it will hang forward of the forestay by about 20% of its length when set horizontal. This can be lengthened even more for a large spin or shorter for narrower spin.
So for want of better advice get a slightly longer pole and attach fittings and try it If he thinks it is too long then it can be shortened.
Actual wall thickness again is a compromise as light wall thickness means lighter pole while greater wall thickness is stronger.I say go for something light. (certainly not scafolding thickness.
good luck olewill