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I feel like your instincts are probably correct, do it as soon as possible and don't do it through text. also emphasize that you wouldn't want to do such a conversation through text.
Accept that you have limited control over how this person reacts to the information, but that you have the opportunity to be an honest, caring, and straightforward person; and fuck it, doesn't the world need more of those all the time?
Well fuck, dude, you didn't mess up that badly, don't get a complex over this. First of all, I wouldn't even say that you messed up you just did something you didn't intend to do (that's going to happen). You thought you had a skin tag and fucked someone without the considering the possibility you had something else or that it was transmissible, which is understandable as you are not a doctor, and frankly (I'm assuming you're American) there isn't much education about this issue or sexual health generally. It sounds like you've got a minor case of HPV, and it's unlikely that your friend will have a major case of HPV, and also a large percentage of people have this condition or something similar and we're not helping anybody by stigmatizing it. I know this stuff can be really freaky because it involves your genitals and people that you're very much interested in, but you'll be fine. Remember when that doctor told you a third of people who fuck have this condition? You've literally passed people on the sidewalk or highway who have been in your position. They're fine, you'll be fine, your friend will be fine. Just schedule a talk or meeting via text and follow through with a honest dialogue. Say that you didn't suspect you had a STI, but that once you learned you needed to do due diligence and inform them.
It's uncomfortable and I'm not advocating that you become callous to these things, but part of having people in your life involves inviting the possibility that you'll hurt someone or be hurt in turn. That's what people do when they get close; this is an opportunity to develop compassion rather than a neurosis, because that would be the most counterproductive thing you could do ever. Sorry for ranting, I've been drinking.
Depends if they've seen that episode of Girls or not, also maybe gays and punks feel different about this stuff, but it's honestly possible she'll not trip. Can't imagine she'd feel better if you didn't mention and had sex again so youre doing right by mentioning
But yeah HPV is super super common, like common enough that she might already have it, if you're swimming in similar circles, and really mostly totally harmless to women or men
Situation sucks, but you didn't do anything evil. Be aware of it going forward and be understanding of whatever response this person has, but don't treat yourself like a demon. You're human and you were working with the information you had at the time.
Personally I'd communicate this kind of info over the phone or in person and wouldn't send over text. I'd say through text that it's some important shit you feel like you need to discuss over the phone, even if only for a few minutes.
If either of you is 45 or under in the USA, you can get an HPV vaccine if you didn't get it already. Good even if you've had HPV to protect you against some types of HPV you didn't get yet.