Putting aside the actual policy issues for a moment, it may be instructive to look at how people might emotionally react to this exchange. Progressives, for example, might feel the need to somehow jump to Bill Clinton's defense and deflect the criticism back to George Bush somehow. But his quickly turns the discussion away from the original worthwhile topic and focus on an emotional defense/criticism dynamic of various politicians. At that point, the civil discussion is in danger of going rapidly downhill.
Instead, a much better response would be to simply say "I'm not here to defend everything Bill Clinton did. I'm sure we can both agree that he made some mistakes, and that repealing the Glass-Steagall Act was probably one of them". Then turn back to discussing the economic issues and policies, looking for areas of common ground, or better yet looking for things where you can learn from each other. Think of this as verbal jujitsu - don't take an attack head on, deflect the force away and use it to your advantage to achieve your goals of maintaining a civil discussion.