In that case she's going to start with surrenders, that being the first thing he brought up. The single most important point to be aware of — he should be familiar with this if he's read the Lastwall handbook, but it comes up frequently with Mendevian crusaders and foreign adventurers — is that if he makes specific commitments, he needs to then follow through on those commitments. Sometimes crusaders whose grasp of Law is particularly shaky will have the 'bright' idea to promise to spare cultists if they surrender, and then not spare them; this is absolutely not allowed. Relatedly, he is not authorized to commit to sparing cultists conditional on them surrendering.
He is responsible for ensuring that allies under his command abide by the commitments he has represented himself as having made. Recommended practice is to communicate to his allies those commitments, the fact that he expects his allies to abide by them, and the steps he will take to enforce them. He would straightforwardly be obliged to intervene if one of them refused to accept a surrender that he had represented himself as willing-to-accept; she's not confident in the full range of recommended options in that scenario, but she's heard "yell at them to stop" suggested as acceptable in cases where the ally just straightforwardly missed the surrender, and "trip the ally" suggested as a usually-nonlethal incapacitation method. (If he intends to employ the latter option, it is strongly recommended to communicate this fact to his allies in advance.) Irabeth's best guess is that the theologically orthodox thing to do with the assassin was to inform him in advance that Blai intends to accept surrenders and communicate the circumstances under which he would interfere with the dwarf's assassination, with the understanding that the dwarf might then choose not to assist him, and to take reasonable steps if available to communicate to the enemies that the dwarf is not in fact under his command and thus should not be expected to hold to Blai's commitments. (The standard mechanism here is uniforms. He probably cannot acquire matching uniforms for his team, though now that it's come up it occurs to her that that might be an actually-useful application of this golem that was donated to Kenabres a century ago by some country in Tian Xia, which is capable of casting various illusions on gear.)
While she was in Lastwall, she heard a sermon arguing that if you're fighting an opponent, and someone vaguely on your side attempts to surrender, and someone on your opponent's side puts themself at risk to stop one of their allies from continuing to attack your ally, you should attempt to avoid deriving an advantage against the opponent who tried to stop their ally from attacking yours. It was a great sermon but she thinks it is highly unlikely to come up under the present circumstances.
Policy in Mendev is to not accept surrender from demons, even unconditional surrender, unless they specifically want to interrogate them; they don't have ways to hold most species of demons safely, and 'surrenders' by demons are nearly always some form of trap. Irabeth understands this to be religiously permissible so long as they take reasonable steps to ensure they don't benefit from anyone mistakenly assuming that they would accept a surrender from a demon.
Questions?