Is it permissible to mention what happened to him regarding seeing an angel and its speaking to him, or are there reservations about mentioning this story?

If what the person told you is in accordance with Sharia, then it is permissible for you to say that you met a person and he told you such and such. As for your saying that you met an angel, it was previously explained that it is difficult to definitively assert that the one who spoke to you was an angel.