Having had her fair share of names thrown at her about her face, it would not have even occurred to Marietta to insult somebody based on their appearance, still less a customer.
She was about to laugh at Severus' comment about haircuts not stopping him from being abrasive, when he went and mentioned being dead. Of course, Marietta knew he was dead in the real world, and she knew he wasn't the only one. Although she hadn't been at the battle, she could remember finding out about all the people who had died. However, it was one thing to know that and another thing to know quite what to say when he was so casual about it. Marietta was silent for a few seconds, busying herself in hair, and luckily Severus spoke again and she was saved from having to come up with a polite comment about your conversational partner being dead.
"Not at all," she reassured him. "I can wash it for you here and show you the best way to do it and what to use, and then you try it at home. It may take a few days for your hair to improve, but it should do so. Let me know if it doesn't and we can try something else."