I would also do a salad, because that really goes best with lasagna (more so than another veggie, although I would do a veggie before I would not have anything else on the side).
I have seen people serve a salad without mixing it. Like have the lettuce in the big salad bowl and then have small dishes or bowls with things to mix in separately (the tomatoes, onions, croutons etc). That way people can choose what they want/don't want in their salad. Maybe your friend can build a salad he actually would like?