When Drew and I got married seven years ago, we had a small wedding with limited space, so we didn’t extend plus-ones to anyone who wasn’t currently in a serious relationship, including my long-distance pals (all the long-distance guests were friends with each other, so I knew no one would feel as if they were traveling alone). Two of my long-distance friends reached out to me to inquire about the plus-one situation, whether they could bring a date or what. I was not offended at all, as it allowed me to better explain some details and make some suggestions. In the end, every invited guest came, I think everyone had a fantastic time, and no feelings were hurt (that I’m aware of). I’d suggest you, too, reach out to your friend and say something like: “I got your wedding invitation and I’m so excited for you! I was hoping you could clarify whether John was invited, too. If so, we’re looking forward to planning a mini-vacation around your big day.”
Your friend may tell you that of course John is invited and she thought that was understood since you’re a couple of five years who has lived together for three (!). Or, she may tell you that, no, John isn’t invited, in which case you’ll have a chance to explain to her that you aren’t sure you’ll be able to attend the wedding without him. There’s no guarantee she won’t think you’re rude for skipping her wedding…but you’ll think she’s rude for excluding your longtime, serious boyfriend from her wedding, so you’ll be on equal footing when you decide whether and how to continue investing in this friendship.
If you have a relationship/dating question I can help answer, you can send me your letters at firstname.lastname@example.org.