tell her before she gets too attached.  get others to support  you and let your friend know you are doing it OUT OF LOVE.  At least  you will open her eyes and make her aware so she will hopefully 'see it coming' before getting in too deep.

And tell her to read the "Paper Bag Princess"

Trishcuit is right. You should definitely tell her, but let her know it is out of genuine love and concern. It's better to be safe than sorry.

 I would say it depends on how close you are to your friend.  Especially considering you haven't seen it personally.  If you're close I'd talk about it once, then leave it at that until you see something yourself.  Just keep your eyes open for warning signs.

I've been in relationship that wasn't unhealthy, but I found out afterwards that a lot of my friends didn't like my boyfriend.  I kinda wished they had said something sooner!  (They really didn't like him, it wasn't them just being supportive after a breakup.)

Even if the guy is a total jerk and you see it firsthand there isn't much you can do.  I've been there, told a good friend exactly what I thought of her boyfriend, listened to hear for a looooong time complain about the situation.  She even would say things like 'I'm one of those stupid girls who just won't leave a bad situation'  *sigh*  I would just look at her and say you know what I think of him already, so there isn't much else to tell you.  There wasn't much I could do other than listen, be supportive and throw a little celebration (but still being supportive that ending a relationship is tough) when the relationship was finally over.

 It is her choice wether she keeps seeing him or not but now she will not be wearing her rose colored glasses. You don't have to go into gory detail about what you have  heard, unless she asks for it. That will seem like character bashing and back-stabbing etc. But don't beat around the bush either.

When I first started seeing the man who became my first husband someone said "Him? He's a major alcoholic.".  It didn't seem that way at the time because it was early on and he was still on his best behavior so to speak. I should have listened.

well i would say if she is a true friend too you then there should be nothing stoping you to tell her. If she was probaly in your shoes right now she would prob do the same thing! so i think the right thing to do is tell her before she gets hurt.

Thanks to everyone. I am going to talk to her tomorrow.

Would it not be possible to arrange for the friend who told you about the guy to talk to your friend who is dating the guy?  That would be best.  Then, if she becomes angry, it won't be at you, and she is much less likely to wonder if you're just trying to make trouble.

Greetings!

Are you certain that this information is correct . . . is it something you have witnessed first hand?  Have you talked to your friend's step-sister or other friend to find out what happend?

You are a caring friend to want to protect your friend.  That is a good thing.  You must prepare yourself for the fact that your friend might not receive your concerns and might choose the boyfriend over what you tell her, and possibly you.  You have to decide if you are willing to risk your friendship with her by trying to protect her.  If you are certain he is a danger, then you may want to risk your friendship to save her from something harmful.  That is why it is important to make sure you have facts (and the correct facts) when and if you decide to talk to her. Even if the facts are accurate she may choose to not believe them and stay with the boyfriend and cut off ties with you.

Keep us posted.

Hope, peace, joy, love . . .