Illness was something Stahma didn't have much personal experience with. Castithans did not become ill easily, they had advanced immune systems despite looking like albino humans. However, she did know how illness and disease could effect people, she'd seen more than one plague in her time on Earth. Though with Ukitake's admission that it wasn't necessarily illness so much as nightmares and memories plaguing him.
Feeling sympathy for Ukitake, and knowing how nightmares and memories had the potential to haunt someone, Stahma wished there was something she could do for him. She stepped closer to him, standing a bit too close because personal boundaries as defined by humans was something she didn't care about. Typically she would invade people's personal space to make a point or make them uncomfortable in a very passive-aggressive manner, but in this case, she approached with sympathy.
"I am sorry to hear that. Nightmares and memories have a way of doing that. May I offer you some company for a while?" She wasn't certain if he wanted company or not, but Stahma put the offer out there.