Andy's point is important though - the member needn't have created a new account.
"If she wants to change her GMail address to an Outrlook address, there should be no need 'to join again'. ?Simply ask her to login with her GMail address and then view her account details and in the Login tab, change the email address. An email will be sent by Groups.io to the new address and she will need to respond to that. Once that is done, Groups.io will transfer all her subscription settings to the new address. ?This is described in the Member's Manual at /helpcenter/membersmanual/1/understanding-groups-io-accounts/setting-account-preferences-and-viewing-account-information?