I have exhausted all my friends and BMs in trying to find a solution to this problem, so I am hoping some of you seasoned vets may have some suggestions! I am not new to the Knot, but I am new to the forums, so please bear with me

Our wedding is one month away, and we have just about collected all the RSVPs from our guests. Like many others, we are allowing +1s for our friends who are married, engaged or in a serious, committed relationship. One of our invited friends broke off her engagement about a year ago, but has been dating a new guy for a few months, a guy that we've never met. We invited her as a single – she is friends with a very large number of the other guests, so she will definitely not feel alone. Not long after invites went out (she was invited as a single), she was talking to my FI and other friends about the wedding and said that she told her new BF that he had to take off work for our wedding. My FI didn't say anything, and planned on addressing it with her later. A few days ago she emailed my FI saying that her BF would not be able to make it to the wedding (even though he wasn't invited in the first place), but asked if she could bring a good (guy) friend who my FI had met ONCE instead.
I have no idea how to handle the situation. If we had invited her +1, then her bringing someone else wouldn't be such an issue. I'm not sure how to go about telling her that 1) she can't bring some random guy, and 2) we didn't invite her BF in the first place. It will be especially awkward because almost all of her other friends that are coming were invited with their BFs or GFs (all of whom we know and are friends with).
Out of all our guests, she is someone who I never expected this from. She has been in a number of weddings and is very in tune with wedding etiquette. On top of that, on all the replies we included numbers, so people would be less likely to fill in extra guests (for example, hers said "number of guests attending: ___ of _1_")....so there is definitely no room for confusion as to who was invited.
Any advice or suggestions on how to handle this? Or how to approach her? I really, really don't want her to bring some random guy....but I have a feeling that to avoid an awkward conversation, my FI will want to allow it....
Thank you!