As a lefty, tell him to not worry about ambi anything and learn the manual of arms on pistols as is. This way, if he’s trying a new gun, or someone lets him try something at the range that isn’t ambi, he’ll have a better time. It’s a right handed world, there’s no getting around it.
I went VP9 first but just learned to figure it out and sold it. The only real issue is safetys like 1911’s can be cumbersome if not ambi. Mag release is index or middle finger, slide stop is right middle finger over the top.
Revolvers are actually less lefty friendly than some might think. The way I was shown to handle revolver reloads involves two right hand fingers through the open frame to reload with the left hand and then transfer back. Only platform I actually have to switch hands with.