how to preserve p v c pipe

Paint it. That will protect the PVC from the UV light.

The PVC will get brittle but it is not going to crumble to dust. There are PVC arbors around here that have been in the Florida sun for 20 years or more. As long as you are not trying to bend it, you will be OK. If you used the gray "electrical" PVC it will be OK for decades. Like the others have said a coat or two of paint will reduce the UV damage.

He said it was going to be an arch!

I assume he will bend it when it is new and not try to straighten it out later. It will take a set and be bent forever. So what?

So? He could make an arch without any bending by using short straight segments between the appropriate number of elbows based on their angle.

I'd say that if you start out with UV-reistant pipe and fittings (PVC-UVR) in the beginning you should be home free. If you can't find the proper stuff locally, try for outdoor-rated PVC conduit which will certainly stand up for decades and is much more easily identified as outdoor material by its grey color. Paint whatever you choose with a plastic-rated spray to achieve your desired color.

I hadn't thought about painting our pvc arch we installed in our garden. We zip tied 1x2 inch cage wire overlapping the ends to our pvc arch. I had wondered if that would be enough reinforcement to strengthen the pvc. It's too late to paint the pvc now, though.