That person's first action, on hearing that Carissa is on her way, will be to swap out his +4/+0/+4 headband for a +0/+0/+6 headband. That should be maximal attainable previous-Kelthamness, and the less different he seems, the less it will hurt her, probably. He doesn't want to hurt Carissa without a reason.
Tarnish will probably have said something cutting enough, or the Osirians warned her enough of their own perspective, that Carissa will not be entirely unprepared; she should have been warned in a context where she'd hear the words warily rather than them hitting with all the force of reality straight out.
(He couldn't ask Ri-Dul to say anything here. Ri-Dul made it a condition of his employment that Keltham's personal life drama would be causally screened off from him to the maximum feasible degree. Which, valid.)