Hey @Aleks_Tofsla - thanks for the question on behalf of your invitee!
I know that when an event is cancelled or rescheduled on my own Google Calendar (for example), it is not automatically removed due to my own settings I have in place within my Calendar. Instead, it shows as a different color (per the calendar settings) with a line through it, indicating it’s been cancelled. Then, the new time/date appears as normal as well. So - this could be due to their calendar settings!
That said, I was able to find a topic in Outlook’s community where others are experiencing this as well, and they had some helpful resources to provide. Take a look and see if this helps your invitee!
Otherwise, I would suggest having them reach out to Microsoft support here.