Influence can go in both directions. Just as she might influence you away from Torah observance, you might influence her toward it. Maintain the friendship, as it is obviously valuable to you, but monitor yourself– does she put you in awkward situations or is she respectful of your lifestyle? If you find yourself starting to slip because of a comment she made or wishing you could do the things she does, that might be a good time to consider ending it. But for now (unless you have had these thoughts already) there is no need to break off ties.
One more thought: in terms of influence (generally speaking), I actually think that somewhat religious people or those who keep some things are more dangerous than those who are completely secular. After all, how does a person go off the derech? Does he suddenly decide he is done with everything? Maybe in some cases, but usually it is a gradual process– he rationalizes one thing, then another, and eventually everything is gone. Therefore, secular friends are really not a problem because their lifestyles are so drastically different that they are unlikely to influence you. Those who are some degrees less religious than you, but still observe some things, may be more powerful tools of the Satan. I still do not think, though, that you need to cut off ties immediately with this person as long as you maintain a self-awareness.