If your Fastrak beeped, I would expect nothing to happen except your account will be billed as normal. If it didn't beep, then your friend would get billed the regular toll if it is a first violation, higher penalty if multiple violations. If you are concerned (and can remember when you passed the gate), you can call the number to ask, they will probably be fine with it and possibly add a note or register your friend's car to your device.