A complete solution will likely be somewhat more complicated than the one provided in another response. Apple has no simple merging procedure, nor one that works 100%.
Assuming she has nothing of importance in the new account, it is best she stick with the original account since that contains things that may not successfully transfer. In that case she should sign out of the new account on the iPhone and sign in with the old account. When it asks her if she wishes to keep the data from the previous signed-in account, say no if there is nothing worth saving, otherwise say yes. That will "merge" the accounts as far as possible. However, some things won't transfer, such as any apps she may have purchased. (I'll add here that it would have helped a LOT to know if she had anything of value from the new account.) If she does have apps purchased with the new account and she does not want to buy them again, she will have to set up Family Sharing between the two accounts. In addition, depending upon where any games, etc. store data, some may be tied to a particular account and there is no way to transfer the scores.
If she has been using iCloud, some items such as photos may have been optimized (reduced in quality). For example, photos taken with the iPhone while using the new account. Before doing the log-out step turn off optimization so the full quality versions of the photos are retained on the iPhone.
You can delete the user ID but it is not integral to the merging procedure. You may wish to simply remove any critical personal data from that account and keep it for a while before deleting it, to make sure you got everything.
