My daughter had her nrotc officer interview today. Back in the day when I went on college interviews, I sent a handwritten thank you. Which medium is preferred for a thank you in 2017, handwritten and mailed, typed and mailed, or email?
Handwritten notes are different from today's norm, and thus may be remembered more. Nonetheless, an email is also just fine. The older the interviewer, the more likely a hand written note will be appreciated.
My suggestion is to let your DD be herself. If she accepts your suggestion to send a written note, then great. But if she wants to stick with email, don't make a big deal about it.