Davelias12:

Why should she forgo inviting the people she wants to come to her party in favor of SOs she doesn't know?

She doesn't have to. But if she doesn't, chances are she won't see very many of the people she wants to come because people don't want to go to parties hosted by someone who has snubbed their significant other.

Davelias12:

Also, she noted that she does not see these people very often, so, most likely, it's going to be an evening of rehashing old times, events, stories, etc. that the SOs were presumably not a party to and not familiar with. Wouldn't the SOs feel left out anyway?

It depends. Personally, I love hearing stories out of my husband's past.

Davelias12:

And, she's the one throwing the party and wants it to be a sit-down dinner, shouldn't she be allowed to throw any kind of party she wants?

Sure. But, she has to realize that there are specific social conventions for different types of parties. If she wants to be "grown-up" and have a dinner party, then she has to realize that grown-up dinner parties involve inviting people's significant others. To not do so would be considered rude and a snub by many people. So, she's free to extend the invite and demand that no one bring an SO or a date, but she's probably going to end up with a small party.