This sounds a bit like AnnieM's Playing Series. It's not so much a party as it is Paul inviting her out on a double date to catch up. Lois needs Clark to play the part of her boyfriend.
In the later stories in the series, there is indeed a party-type event in which there's a confrontation between Paul and Clark. Hope this is what you're looking for. If not, you should read these anyway.
Here are the links for the stories in the order they were written.
Playing the Part Playing to Win Playing for Keeps Playing by Heart Sara
