So I just went and checked out the specifics of this battle, and the key phrase is “speak with empathy and compassion”. If the player doesn’t approach the character in question with actual “empathy and compassion”, then they don’t get to make the persuasion check.
If your party actually figures out that someone needs to talk to this guy, and HOW they need to talk to him, then it’s perfectly fine that they automatically succeed on the roll, as long as they let the most eloquent person in the group do the talking.
I imagine most parties will go into default “boss battle” mode and not try diplomacy at all. If you want a party to stop shooting, smiting, and casting, and instead start talking about feelings, in the middle of a boss fight, that’s going to probably take some pretty big hints by you that it’s even a possibility. If they figure it out, then they deserve to win that way.