He's harassing my friends. What can I legally do?

Yes, that sounds like the definition of stalking. If she hasn't already, she must make a report to the police. She needs to keep a record of everything that happens however minor it may seem, including dates and times, and witnesses to any events. If this goes to court, having such a log will be hugely beneficial to the prosecution.

She can apply for a restraining order to keep him away from her physically, and to prevent the phone calls/emails etc. However, really getting a stalker to stop is very difficult.

The big problem is getting a stalker to understand that the relationship is really over. Through fear or kindness, people often leave some kind of hope dangling that the stalker grabs on to. She should make sure in any contact that she is forced to have with him, she is VERY clear:

"I do not want a relationship with you. I do not want to see or talk to you. I do not want you to contact me directly, or try to contact me through my family or friends. I have no feelings for you."

YOU cannot do anything- your friend is the one that has to get restraining orders, etc., against the harasser.

Either contact your sheriff's department or police department and see if you can get them to help you. If they won't (they may not if there is no physical evidence such a bruises, etc.) then you can go to your local muni or superior court (probably in civil or family law division) and they can help you get the proper docs to get a restraining order/hearing. They may even have a TRO (temporary restraining order) clinic you and your friend can attend where volunteers help you fill out the necessary forms to get an Ex Parte (emergency) order pending a permanent hearing.

What state are you guys in?

YOU as the friend of a friend can legally do nothing.. If theres any action to be taken its gotta be the person the harassment is against (yer friend)

YOU arent being harassed (though the instances you cite are likely not gonna be considered harassment legally unless the Faimaly & friends consider it so and file charges themselves)

Legally? YOU have no standing in the matter.. not even a little.. YOU can do nothing..

you personally can't do anything legally except report to the police in case something worse happens - your complaint can possible be used against him in court.

However she can definitely at least get a restraining order which means he'll be violating the law by calling her or coming within a certain distance of her.

so once she has this - if he calls her or comes within 100 yards (example) she can immediately call the police and have him arrested.

He could be classified as a stalker. There are several anti-stalking laws in many states.

I'm not sure what the criteria is, but I'm sure that if there's enough evidence, your friend could obtain a restraining order to enjoin her ex from contacting her.

Easiest one and most satisfying is for her to move to Texas. The her to buy a shotgun and gun the bastard down when he tresspassed like all stalkers do. If his hands are open it is legal to gun someone down if the tresspass. This is in Texas I don't know about other states.

he's definently a problem. First thing would be try to get a restraining order, then go from there. Please check with your local police department before he finds her.