Recommendation from institutional email or private?
You seem to have answered your own question. If you don't want the recommendation to be subjected to further review, you should ask her to use her institutional email address. Anyone can set up a Gmail account, whereas there is a level of authenticity implied by an institutional email address. It's not relevant that she isn't sending the letter in her capacity as a professor, since the main purpose of the email address is to authenticate who she is. The college are requesting that letters of recommendation are sent from institutional email addresses since it reduces the amount of extra work required of them and of the recommender.