There's a good chance she is as embarassed and mortified as you and your wife are.
Perhaps your wife could ask if she wants to talk about it. Keep an open mind. Suggest (strongly) that she needs to be on birth control if she isn't already.
This is just something teens (and adults) do. It is however, meant to be an intensely private and intimate moment.
This will be an event that you will all remember for the rest of your lives. Don't make it worse than it already is.
Think how you would feel if something like this had happened to you and your wife when you were young. Above all, you don't want her to be scarred for the rest of her life because she was caught doing such a private act. After all, there is no shame in the act itself, but the circumstances are awfully awkward.