Firstly, your girlfriend has her light mounted incorrectly. If at all possible move it further back on the bike, so that the tyre and frame don't block the view of it. If the light is designed to be mounted there it's a poor design.
Having the big block of colour/reflectors on the road side sounds like a good idea. But the pannier will still be visible on the other side of the bike, so it's more a matter of having it slightly further out. That also means the clean side of the pannier is facing traffic turning from the other side of the road, helping potential T-bone drivers see you.
I think it depends on whether you're buying one pannier or a pair. Generally people buy panniers in pairs so they can use both when they want to carry more stuff. And you want the pair to wear evenly, so you don't end up with one ratty old worn-out pannier paired with a nearly new one. So it's best to alternate the panniers from time to time.
If you buy symmetrical panniers this isn't an issue - you can keep using your single pannier on the outside, just swap them over occasionally (so the former "left" side pannier now sits on the right, and vice versa). With larger rear panniers they're often asymmetric, though, so you can't do that. I'm not sure the advantage of having the pannier on the traffic side is so great that I'd be willing to wear that pannier out leaving the other side one much less worn. Maybe buy two of the traffic side one?