The Bard player still has to say the right things in the first place to make the check at all, there is not guarantee they will do that. But if they do... LET THEM this is literally their characters big thing! Why not let them do that specific thing? It feels backwards to me to have a "Well this fight has a conversation skill option but I am not going to let the character that has conversation skills participate".
IMO the rolls are not the interesting part of this, the party figuring out how they can get bad the guy to change their mind is.