Gmail address associated with Outlook on new phone

My wife uses Outlook as her primary email account and Gmail as secondary.  She just purchased a new Samsung Galaxy phone, and we transferred everything from her previous phone to it.  However, when she sends emails through her Outlook account on the new phone, the sending address is her gmail address.  I tried to change this by going to her Outlook profile and adding her Outlook account, but this option was linked to Google which wouldn't accept it.  Does anyone have a solution?  Thanks.