Being cruel to her won't make her decide to leave if she's made up her mind to stay. It'll just make it harder on her.
[he could offer more thoughts, like, "an apology is a traditional place to start," but he actually doesn't want to make it about Saber. he's sure that she'll be back, and Zelgadis has had a tough enough time]
...But... there's probably no point worrying about that right now.