Wow...that's a tough question. One which definately deserves some thought.

I think that answer is no, HOWEVER that solely depends on how you approach this. I feel there are two possibilities of handling this, (the latter, imo, more favorable for ALL involved).

Option #1 (this is very similar to your chosen option #2, however, hastened).
Abandonment in any form can be devistating, especially for those receiving, but also can be traumatic if you are forced into creating it.
So, with that as fact and due to her emotional/mental state, just "leaving" her where she is at present may not be the answer (for now), however, distance MUST be immediately applied and communications immediately brought to an absolute minimum.

Of course, offer her a complete explanation as to why you have reached your decisions of distance, and reassure her that you are not "abandoning" HER. You are attending to your priorities that you had previously allowed to be jeapordized by your involvement with her and had lost sight of.
Reinforce to her of her importance, but also the inclusion of yours.
Do your best to instill upon her that there is no sound future with you. PERIOD.
If there was physical involvement, DO NOT venture there again, (this would only be a contradictin to the message you are telling her, confusing her even more).
Remind her that nothing is more important to you than your family, and you will not allow anyone to ever jeapordize your family structure, yet, emphasise her importance (as a person) as well.

Think thoroughly a "schedule" of any future encounters with her, informing her that said encounters WILL become progressively fewer as time progresses, (and said time must not be initially extensive), and eventually, to a closure....as you ARE, after all, married and committed to a life which you CHOOSE for yourself.

You must quickly ween her of her need and dependance of you, while reinforcing her sense of self-worth and self-gain, as well. NOT an easy task, as she is suffering from serious pre-existing issues.
Keep in mind, too, that the approach of #1 may more than likely, (though I'm truly hoping not the case), not result favorably....Not because of your efforts have failed, but because she may not be capable of being receptive.
This approach may lead to you down a path of continued guilt. However, it is still an option.

You certainly do have an choice here, and perhaps the next suggestion just may be a means of attending to this without creating any further guilt for yourself, and even possibly eliminating existing shame, as well.

Option #2
Due to the fact that it is because of your love for your wife and child, you are, of course, guilt ridden. This behavior of yours is what has violated your morals and family structure that you have committed yourself to, and have since jeapordized.
Because of that, and because that is the source of your guilt, perhaps if you were to confront your wife with what is going on, share your (genuine concerns) about the welfare of this girl, this will allow you to attend to what matters most to you, wife and child, as well as maintaining a distant support for this girl.

To what extent you wish to divulge the entire scenario with your wife is entirely up to you.
I have no idea how sound the structure with your wife is, but from the sound of it, it is quite strong. Have faith in that bond with your wife. Include her on your plight to "guide" this girl. Who knows, your wife just may prove quite beneficial to your initial goal.

It just seems that, in order for you to truly help this girl, it cannot be at a sacrafice to you.
Guilt is eating at you, distorting your judgement, imparing the quality of assistance you are to this girl.
Eliminate the guilt. Include your wife. Combine your energies, and you are surely to succeed....for ALL concerned.

Also, all the while throughout this, do not forget about your own self. Forgive yourself. Be your own best friend. Allow other's to help you help others.

All just fragments of my humble opinion..lol.

I hope the best for you, miragex. I'm confident that you will choose the right path and do what's best for all concerned.

Again, keep us informed. We do sincerely care.

All the best.

Shangrala