As soon as her MileagePlus number is on the reservation, she will be able to see the trip whenever she's logged in to united.com or the United mobile app -- and she may occasionally get e-mails about the exciting in-flight entertainment options for her upcoming trip. And there will no way to remove the trip from the logged-in view on united.com or the mobile app once it appears.

If you book the trip for her as a "new person" and enter nothing besides her name / birthdate / known traveler number (and if it asks for address/email/phone, just provide your own), there is a fairly good chance that they won't figure out who she is and ruin the surprise.