“But you can’t-”

Yes, they can.

We have all experienced incredulity or disbelief at someone’s actions - the actions of someone we love, or maybe someone we don’t know that well at all. And many of us have probably further had the experience of telling people in our lives what they “can’t” ever do again. For better or worse, the other person absolutely can do whatever it is they want or choose to do, and us telling them they “can’t” will not stop them. In fact, it may imply or foster a power imbalance we do not want in certain relationships - those we have with a partner or close friend. Just because someone does something we do not like does not mean we have the right to deny them their agency.