Force at second if batter is out at first?

[BDevil15]

Ok, I am very new to softball coaching but played baseball like 40 years ago so forgive me. Does anyone have a rules citation for this play?

Runner at first with no outs. Runner takes a small lead and the ball is hit sharply to first. First baseman steps on first to put the batter out. Runner heads to second and slides into second just after the ball arives. There was no tag. Is this still a force when the runner behind her is already out? I though that she had to be tagged but was completely overruled. I can't find this specific situation in my brand new rulebook so hope someone can help.

Thanks,
Coach B