Positive ChoatieKid did not hand write anything (Good Lord! His penmanship marks him as a serial killer). I doubt he sent e-mails either. I’m with those who say notes don’t affect admissions but could be viewed as good manners, a lesson entirely lost on my kid.
My kids were advised by their MS to hand write TY notes to AO. Email is fine for student tour guides.
One has bad handwriting, but can do okay if focused (and kept it very short).
Speaking of bad handwriting, when one of my kids applied to Exeter, a portion of the application (essays I believe?) had to be submitted in handwriting. That was torturous!
We were advised to hand-write thank you notes to the interviewer, and to email the student tour guide and any teachers or coaches that we met. After that initial handwritten note, if applicants had follow-up questions or any other need to communicate with their AO, they could email. Honestly, I’m not really sure that it matters but we did it old-school anyway.
I was once a stickler regarding handwritten notes but I have softened a bit over the years. My inner Emily Post is screaming at me right now…because I do believe it’s important for kids to get into the handwritten note habit for certain occasions. If my kids spent a special weekend/ vacation with another BS family they would always send a handwritten note to the parents. If a teacher went above and beyond for something… they’d pop a handwritten note into their box. That sort of thing. My proud moment came recently when K1 received a wedding invitation without a response card enclosed and he knew what to do and helped out his roommates who were bewildered.
I can die now.
Either way- ( at least at the schools I’m affiliated with ) it goes into the file along with the app and everything else for the admission team to review. So, in my mind, how it gets there is kind of irrelevant as long something gets there. I know when I do off campus interviews as an alum, I pass along the thank you emails/ notes I receive from candidates to the AO and if a candidate sends an email/ note ( mentioning me and our interview- usually when someone loses my card… or is confused about what to do ) then they forward it or scan it and share it with me. It’s all very nice and the notes I’ve received were all super and truly appreciated.
FWIW- At JBS, a handwritten note to a SS AO is mandatory. No emails. A Thank you note to an AO via email is considered a capital offense. A longtime ago… when I fell on ice ( several times during a tour with K2… ) , I sent a note to the infirmary nurse who bandaged me up. Whatever you’re comfortable with is the right thing to do . … but I am a sucker for a handwritten note. A simple white or ecru fold over note and a fun stamp makes a nice statement and it shows that you took the time. 
I would personally suggest emailing them to your interviewer. It’s unlikely that you will get a response over physical mail. Also, I am not sure if they read the written mail based on the way the mail system works at boarding school. I was accepted by one of the schools where the interviewer emailed me back. There may or may not be a correlation though. I kept mine short: three-four sentences.
Actually… most AOs reply to a handwritten note with a handwritten note. Not always but most of the time. If you send an email you’ll likely receive an email in return… it varies by school tradition , personal preference and style.
Also a thank you note doesn’t usually require a response. It’s just a thank you. That’s why I said to email later in the process if you have any specific questions, but hand write the thank you note.
Most AOs sent handwritten thank you or holiday cards after the interviews. I could not get DS2 to write notes nor emails. He was very stubborn about it, so I let that go. The application process was onerous and stressful enough. sigh.
I could’ve wallpapered a bathroom with all the thank you notes and holiday cards my kids received…
Don’t worry about it. She should send something, because it’s the right thing to do, but I doubt anyone is paying too much attention. FWIW, my son sent brief and cursory emails and I doubt that was held against him, while the heartfelt handwritten letter (not a thank you note) my daughter sent to one school wasn’t even acknowledged with a cursory email in return.