Red is generally thought to be better for preserving night vision, as the rod cells in the eye are MUCH less sensitive to it, so using a low-level red light will leave your rods pretty much undisturbed.
On the other hand, red provides terrible color rendering, so it may not be suitable for some people or tasks. If you have to look at maps, for instance, many lines disappear under red light.
This thread contains a lot of information relevant to your question, so it may be worth reading (more threads are out there, too, in case you want to search around).
The basic key to a light that will preserve night vision is that it has sufficiently low output. Too bright of a light in ANY color (green, red, yellow-green, whatever), and it can destroy your night vision. You want to choose a light that has a level suitable to preserve night vision for whatever tasks are expected to be performed.
Specifically, I would check with your buddy and see if he'll want to use the night-vision-preserving level for up-close tasks like reading, or if it'll be for searching the general area at 10+ feet, etc. Note that the A2L's low level really isn't that low, so if the tasks are up-close reading, etc., then neither the green nor red version will preserve night vision exceedingly well. (However, the red version of the A2L is supposedly 3 lumens, whereas the green is 10, FWIW.)
