i was just wondering if someone could confrim the rules in this particular area

i was play a friendly game with a few friends when it came to the point were one player could meet his high blind obligation. we were just wondering what happens in this situation

does the player just go all-in and the small blind is met as normal

or

does the small blind only have to be half what the high blind has managed to put in

or

does the player automatically forfeit

thanks to anyone who can clarrify this