This is a problem.
The guest just needs to go to marriott.com, look up the reservation, and will see the owner's full Bonvoy number, mailing address, phone number and email.
I rent out a lot of Marriott and Vistana weeks, and had I provided confirmations many times so that the guest and/or Redweek could verify them before paying. This resulted in my personal information being leaked to scammers and suspicious activity with my timeshare week reservations. Examples:
1. Guest name on the reservation was changed by someone a few days before my guest's check in. It was not the guest who did it. I only caught it a couple of days before check in because I checked the reservation.
2. When checking another (unrented, still in my name) reservation on marriott.com, I saw someone's email address instead of mine in the guest info. Then I saw the same email address in another future reservation. So someone (a scammer?) looked it up on marriott.com and emailed a confirmation to themselves, possibly to defraud someone else.