Yes, it has to be really unless it's a very good massive red hearing! I mean look at the intro, the shadow on the floor clearly has her hair, and then if you go into the room with her at the party, she asked if he STILL likes the Beatles, showing she knows him. What is also interesting, is that if you either kiss Natalie or get the lewd with Sophia, they get a beating in Chapter 2, but if you don't go with Sophie and don't kiss Natalie, they don't, they just get drugged. So that again points to Elly I think.
I guess Elly is stalking him and either follows him to the party if he goes straight away, or sits on the bench if he goes later and then makes up the story up about Mai - as you don't go inside for that option, Mai could already be there. Even if you don't talk to Natalie or Sophie alone, Sophie still posts the photo, which is then Elly's motivation for that. Although she is there and knows for a fact what happened, so it's is a bit extreme thing to do for that only!