Has facebook ever logged a friend of yours in even though they've never logged in from that device?

[Edward]

I went to log into soundcloud from my girlfriend's ipad. I used the "log in using facebook" feature expecting a log in screen. Instead it logged a friend of mine in, meaning he was signed into facebook. I asked my girlfriend about it and she said she hasn't seen him in years. I want to believe her but this sounds absolutely outrageous to me. The only answer I can come up with is that she's lying. I've searched the internet and not one page showed up resembling a similar problem. Someone please ease my mind...