Nothing will be sent to her at all until after the interview, when they send her passport back. For that delivery, you will either give the delivery address or choose from a fixed list of pick-up locations when you schedule the interview. The embassy *might* send her a welcome letter and pamphlet, but it probably won't arrive before she is already out of the country. As was mentioned before, just put her new address on the DS-160 when you fill it out.
Don't worry about updating her address until you fill out the DS-160. You'll be fine.