Your friend would have also seen the "Signed in at two locations" pop-up; and if he/she was payng attention, would know that someone signed in on there account. They may have dismissed it, but chances are, they checked the other location. Your computer name would have shown up yes.
If he/she noticed it was your computer, chances are they will message you.
Your friend can know that someone signed in but he/she wont know who.