I would not accept it. If she asks you can tell her you are rarely ever on there and didn't even see it. You can also ask why she did it at that time. Don't say anything unless she brings it up.
I eventually had to defriend my stbx because it was very painful to see her updates and her pictures with other people when I felt exiled and rejected from her life. It was to protect myself and not to punish her.
If it wouldn't bother you to see a pic of her with OM and you are detached enough to handle things like that then I don't see a problem with it but that's typically not the case around here.
