He's hurting and he might not even want them to get back together or anything. He just wants her to know how horrible of her person she is (I'm not saying she necessarily is, as I don't know her, but that's what he thinks of her). He wants to make her feel...something.

He's entitled to hurt and expressing that pain. I get it.

However, it seems she doesn't care. He sends her the letter and then what? He'd be lucky if she doesn't reply at all. But what if she does and her response is nonchalant? Or even malicious? Then what?

He'd just be prolonging the hurt. Or maybe even exacerbate it.

He just needs to allow himself the time. I'd encourage him to write the letter, sure. But not to send it. When he's allowed himself sufficient time to heal, he'll look back and feel 100% differently.

That sucks. I wish him the best. I've been there.

Edit-- because obviously English is my second language today.