Is it appropriate to ask if you can bring a guest to a wedding?

In the next month, I will be attending the wedding of two friends of mine (Several days ago I asked a gift-related question about this same wedding). About two months ago, they created a closed facebook group for the purpose of getting everyone’s addresses. The date and time of the wedding was posted on this group, but the invitations were sent out about a month later.

Upon reading the date of the wedding, a close friend of mine informed her boyfriend about the wedding and had him rearrange his travel and schedule plans so that he could attend. She has been dating this person on and off for about a year now, and has (until recently) been somewhat secretive about this relationship.

Several weeks later, she received the invitation and was not given a plus one. She initially said she was kind of sad, but understood that this is a smaller wedding and that it probably just wasn’t feasible for the couple to give everyone a plus one. Since then, she’s changed her mind and intends on asking the groom if she can bring her boyfriend and is going to offer to cover the cost.

Is this appropriate? For what it’s worth, I wasn’t given a plus one either (and I’m under the impression most, if not all, people weren’t) and I’ve been dating my boyfriend almost as long.