I think so. But that should not prevent the 9/11 victims from seeking justice, IMO. Justice should be blind, as the old saying goes. As for any future cases, let the chips fall where they may.
As a side note, remember how Obama bowed to the Saudi king? I suspect that Obama will veto this bill not because of your friend's point, but because a lawsuit would expose the Saudi's deep involvement in 9/11.
This would not be a good idea and would really open the US up to similar lawsuits in international courts. In my opinion this is some lawyers wet dream that would drag on for years and the outcome would yield little for the families and incredible headaches for our own people serving overseas.