My fiancé's parents are separated and his father has a girlfriend. His mother has made it clear that she will not attend the festivities if the girlfriend is invited. The father has set his own ultimatum -- if the girlfriend isn't invited, he'll just bring her to the ceremony anyway but won't attend the reception. This really puts my fiancé and me in an awkward position of either choosing one parent over the other or telling neither to come. Please help.
You're shouldn't feel forced to make a choice based on their little catch-22. His parents have no right forcing their son to choose which of them he values more -- geez, it's like a toddler asking his mom, "Who do you love more, me or my baby brother?" There's no diplomatic way to answer that (especially, as is sometimes the case, if you do have a preference.) Issuing an ultimatum was beyond the pale; bringing an uninvited guest is just Not Done. Since the parents won't behave like adults, it falls to you two (ostensibly the kids) to wangle some decency out of them. Unfortunately, their advanced age probably precludes you from being able to bribe them with extra slices of wedding cake.
There are two different paths you can take based on the specifics of the situation.
If the girlfriend is a long-term, serious relationship and his parents will be divorced by the time of the wedding: Invite all three, including the girlfriend, who just might end up being your stepmother-in-law and therefore is someone it's not wise to alienate. Make sure both women know that the other is expected to attend, so there aren't any nasty surprises.
If mom balks and says, "I'm not attending if that little tramp is there," your fiancé should not get visibly upset: instead, he should say, "I'm sorry that you can't set aside your anger at him long enough to be with me on one of the most important days of my life. You'll be terribly missed by everyone." Tough love! Make it clear that even if the girlfriend isn't invited, dad plans to have her in tow at the ceremony anyway, and you can't exactly have bouncers at the chapel door waiting to toss her out on her ear. His mother needs to adjust to the idea that a divorced couple should expect that their former partners are going to be squiring their new loves to family functions.
If the girlfriend is a short-term fling and the divorce will not be final by your wedding day: Explain to dad that this is, after all, a wedding, at which the sanctity of the marriage vows are celebrated. For him to bring a date when he's not even divorced yet is to flout and degrade the very institution that everyone has come to witness. Divorce at least has the sheen of gruding societal acceptance that adultery lacks. Explain that regardless of whether you like the girlfriend or not, or approve of his relationship with this woman, the fact remains that HE IS STILL MARRIED TO THE GROOM'S MOTHER, and cannot under any social convention or rule of etiquette expect that his girlfriend be welcome at the wedding.
If he's at all rational he should be able to see that his son's wedding is *not* the place to flaunt his sexual prowess with other chicks. If the girlfriend is someone whose good opinion is important to you (either because you like hanging out with her or suspect that her relationship with the father may take a turn for the serious, ensuring that she'll be a part of your life for years to come), make some concession to her, such as having cocktails with her and the groom's father after the rehearsal dinner.