Since there aren't any other single people invited, it is doubtful that someone would gripe about a plus one for Daphne.
I still think Greta should speak to Daphne. Let her know that she is welcome to bring a date, if she'd like, but if she'd prefer to bring her sister or a mutual friend, etc. so as to have company for the drive, she's welcome to do that, too.
I was invited to a friend's wedding, with a plus one. I wasn't going to bring anyone because I wasn't dating anyone. The bride then suggested I bring a friend, if I wanted to. Which worked out perfectly; I brought a friend of mine who knew the bride, too, but was further removed and wasn't going to be invited, otherwise.