Is he suppose to pay in advance? If not, showing up late, making a quick appearance without ordering so he can congratulate them and then 'bounce' could work.
If he is suppose to pay in advance, then he needs a conflict. Depending on the group, he may be able to effuse his best wishes and regrets, and just say he is not available. If they are a pushy or nosy lot ( and I suspect someone throwing her own party is pushy), they may not accept this and push for details or offer to change the date to accommodate his schedule.
If he is in a big group, it is easier to not go, but for a small group, or if he has a prominent position, it is harder.
Lots of dimensions to consider...