I’m not trying to shit down your throat, but trying to use kinder words than the other person (I might have failed) to explain why it’s not an ideal comment to leave there.
I’m not saying it’s bad advice, but people generally have emotional responses to what others say. That’s why delicacy is important- if you say the right thing the wrong way it can make the other person less receptive to the idea as a whole.
Of course this is within their control and you’re not responsible for their emotional reactions, but it’s also a pretty consistent, predictable reaction, so being aware of it and accommodating it is a good idea if you want your words to sink in.
There is also the critique that stoicism makes you more easily exploited, if you come to it without assertiveness already well established. In the OP’s case, telling someone who has wrongfully beaten themselves up for years about an interaction where the other person was cruel to them for no reason, that they’re actually the one at fault, is unlikely to provoke a thorough consideration of stoicism. It’ll either lead to them dismissing you and your ideas, or it will lead to them continuing to beat themselves up.
Edit: the current top comment for me is a good example (imo) for how to express similar ideas without making people defensive.