If you think a lot of your friend, why not go for a short period? Have an excuse ready to make an exit. Perhaps get a friend to ring you halfway through to provide you with one.
If you're enjoying the party, you can stay. Friend on the phone can be primed for this eventuality.
You can still go out to lunch another time with your friend.
Water Pollution -“ A National Disgrace”? A case for renationalisation?