I would recommend that you do not make changes to your cross-save setup, as it will indeed mean locking your accounts out of cross-save for 90 days. This means that if you deactivated cross-save right now, you would not be able to use any of your accounts for another cross-save activation for 90 days. It is also not advised to link accounts that are not yours, as that can cause issues of its own. Your friend should be using only their own accounts, if they wish to activate cross-save. They can look into setting up their own Stadia account, if they wish to utilize cross-save to play on that platform. If the intent is to have a suitable gaming PC on hand, then they can simply login to their Steam account, and play as usual.