So I am putting this in for a friend who recently tried to sign in but when he tried to sign in to his account here it signed him into another account that’s not his and no matter how many times he signs out it just puts him back into this wrong account
This sounds more like an Xbox issue than a Halo one. Has he tried signing in over at xbox.com? If the same thing happens there then he needs to contact Xbox Support.
Almost guaranteed he is using the incorrect email domain, ie ‘hotmail’ vs ‘outlook’.
As noted above, for account specific help, your friend needs to reach out to Xbox Support.