(I had to use my マガポケ to read the entire chapter for the full context. The key context is that the girl is basically a troublemaker and he was skeptical of her personality and intent. Otherwise, the sentence in question makes little sense.)
You probably have no problem reading this:
Natto is so delicious that I (even) want to eat it every day.
Natto is delicious to the point where I want to eat it every day.
In this construction, the part before くらい is a (surprising/extreme) result/conclusion based on the associated reason (おいしい). The reason part is typically a simple adjective like おいしい, but in your case, it's a long clause (黙ってくれてる).
For now, she (Mami) keeps the secret about Mizuhara from the office, and for this (reason) alone, I'd rather/even say she's kind.
黙ってくれてる is the reason, and 優しい is the (surprising) result (which he has to admit). The guy (Kazuya) emphasized 優しい with くらい because, up to this point, he had been so nervous and suspicious of the intent of his ex-girlfriend (Mami). While walking together, he started to convince himself that she was not that evil.