I'm not a parent, and only a few years older than your daughter, but I have siblings the same age as your daughter who I've had to have similar discussions with. Feel free to take this with a grain of salt.
I understand that people in this subreddit want to be "sex positive" and tell you that you should allow your daughter to experiment, but I don't think they realize the potential implications that this could have for her romantic future. I'm not saying that it will definitely turn out to be serious, but it could, and as a parent I understand your desire to step in. Just don't blast in and stop her; calmly pull her aside and talk to her.
Be honest with her about how this came to your attention. Explain to her that the only reason you continued to snoop is because you were worried about her, and assure her that you won't continue to invade her privacy without good reason. Also assure her that this has nothing to do with her interacting sexually with a girl; she'll be more likely to get defensive if she thinks it's because you disagree with her sexual orientation.
If it were me in your position, I'd let her know that while she's at the age where it's understandable that she be interested in sex, she's not quite at the level of maturity that it takes to comprehend the potential emotional effects of sharing yourself with another person. Self-exploration and discovery is natural and encouraged, but until she gets to know herself as a sexual being, how can expect to contribute healthily to a relationship? I'm trying to think of a good analogy; perhaps, "how can you allow someone else to tell you how you look when you haven't even looked into a mirror yet?"
If the friend is really being the aggressor here, that can set an unhealthy precedent.
I'm not saying that all 16 year olds are emotionally ready for sex (or that all 13 year olds aren't) but at that age, one is typically more secure in themselves as a person. Not that puberty doesn't cause chaos, but one is more accustomed to disorder.
I'd allow her to continue interacting with the girl, but explain your concerns. What they're doing is relatively harmless, but if allowed to go on completely unchecked, your daughter could pick up some unhealthy behaviors. At least if you discuss it with her, she'd be more knowledgeable of red flags and less likely to allow her friend to influence her completely.
When you're young, it's normal for a new relationship to become "obsessive." Continue influencing her to partake in other activities at regular intervals, but don't make it out to be a punishment. Get her to participate in things in your new community, make some new friends. Don't allow her to become dependent upon this girl for entertainment; it'll open her up to depression when it inevitably ends.
My mom always took the position of letting me make my own mistakes, but with support. That way I wasn't too sheltered, nor too wild, and I think I ended up okay, for the most part. At least I know how to handle my own problems, and I trust my mom to help me if I hit a particularly rough obstacle. Can't say the same for many of my friends.
You seem like a good parent. Don't coddle her; when it comes to adult topics like this, treat her like an adult. Just an adult who still has a lot to learn.