Girl likes me, how can I make it easier on her to say no

So I have been friends with this girl for a few months now.
And, another mutual friend tells me that she really likes me.

Anyway, she only wants a serious relationship (even life-term relationship).

I am not ready and don't want such a serious relationship.
She doesn't know i know, and she told my friend not to tell me anything.
Now she is asking him to "suss me out" etc and find out if I have feelings, would consider a relationship etc.

The weird thing is, she knows that I have casual sex partners and one night stands etc. In fact I think from previous conversations she should realise I am not one for long term.

Just wondering, what is the best way to go around this? Basically as I care for her, I just want her to be in as little heartache as possible. She'll likely never know that I know, but maybe there is something our mutual friend could tell her that he "found out" from me?
Or, if she ever does tell me, what do I tell her?

I'd like to emphasise that what I am trying to do is make sure she isn't hurt

Dumb question from a man-whore.....NEXT!

She's not even asking you this face-to-face. Why can't your friend tell her the truth? I don't see the problem here.

Agree with Giga. Your mutual friend should tell her how you randomly landed on the subject of serious relationships and how you were saying that you still quite enjoyed playing the field and not having to commit just yet. Keep it simple and do it soon cause those things always have a tendency of getting more complicated with time.