What's the responsibility of an employer if an employee is sexually harassing his co-workers?

At my job one of the workers (male) has approached two different women and been lewd and with one put his hands on her hips and humped her from behind. She told her boss who suspended him for one day and the next he is back at work. When the woman asked the boss why the man was back at work he said "You don't have any hard evidence against him". Now all the woman are afraid to be anywhere near this guy. Our boss even admitted that this man was fired from his last job for sexual harrassment. Sexual harassment a lot of times is not seen by anyone else. How would you have hard evidence? What should my co-worker do???

My first thought is; did anyone else witness this action on the male's part? If so, then will they say as much to the boss if asked to? If they will, then they should. The two harassed females should go together and talk to the boss about the problem and mention the words, "sexual harassment" and "hostile work environment." Those two together should perk up the boss's ears.
If that doesn't work and there's no witnesses around when the man does this again, she should reach down, grab his "business" and twist it like she's a tornado. It is, after all, his word against hers... and there are no witnesses to it.

To fire someone you need a well documented case. If this guy does it again he should be reported again.

How would you feel is your old mate came to work where you works and because of a grudge reported you for harassment. Would you want to be fired because of an accusation?

Once the employer has notice of the violation, he must take steps to correct it, i.e. demotion, firing, sexual harrassment counseling.

Otherwise, the employer opens his company to liability in a sexual harrassment suit.

I would recommend having your co-worker contact an attorney for counseling.

If the woman has told the man that she did not like his advances, and he continues to do it, her boss is obligated to put the man on report and warn him that he needs to stop this behavior or suffer the consequences, especially given he past behavior; the woman should check with the company's HR dept to see what their protocol is, and follow it. She should not be afraid of retaliation from the man or the boss.

What Menifee Maniac said is correct.