First off, my appreciation to your friend for his service to our country. regarding the left hand build, two thoughts...
1) Regarding chambering: .270, 280, .30-06. Pick one and go... Good brass availability. Good selection of bullets (Lighter for target practice. Heavier for game.) Easy to reload for if he is so inclined...
2) If your friend is thinking of a true custom build, might want to consider a custom action such as Stiller, MRC, etc. For only a little bit more money than the Rem 700 donor action + machine work to clean it up / square it up / "accurize" it, you can buy a custom action ready to build.
Or... he might consider "pre-built". There are always nice LH projects coming up for sale between here and the other internet sites for significantly less than the cost of the parts...