A.R.
My 8 year old son started putting his fingers in his mouth when he was around age 6 and he had never sucked his thumb; it was just a new nervous habit. We talked to him about where his hands had been, what he had been touching, and how gross it was that he was now eating whatever that was. Constant talking about how gross it was, reminding him to take them out of his mouth, and liberal use of hand sanitizer stopped the habit.
Just gently ask her to take her thumb out whenever you see it, and be consistent in not letting her suck it. Hand sanitizer can also work since it tastes bad. Maybe you could buy her a new stuffed animal to hug instead of sucking her thumb. Since this is a new habit it shouldn't be that tough to break, but don't let her get used to it because thumb sucking is a hard one to break once they get 'addicted'. My daughter was a thumb addict for 4 years, we finally had to get a special guard to break her habit. Good luck!