Reuben co-starred on the show from its second through its sixth seasons, then went to work as a background performer for Tina Turner.
As an HIV-positive physician's assistant, Boulet was the first female character on a prime-time network series to carry on with her life and work while dealing with the virus.
Reuben, a real-life AIDS activist, will play the character on Thursday night's installment of the hospital series, which is now in its 14th season, the Los Angeles Times reported.
"It just felt incredible," Reuben told the Times.
"Of course, there was a lot of apprehension and anxiety, like returning home after not having been there for so many years. I didn't know what to expect. But when I got there, it was like no time had gone by at all."
"I can't think of another situation where you're around after 10 years and you get to revisit something. It was just so great to see her and see how she has matured. The character has matured like she has," said producer Christopher Chulack.