Mack originally pleaded not guilty after she was indicted with Keith Raniere, the alleged leader of a self-help programme which encouraged women to become sex slaves.
According to prosecutors, the actress co-created an initiative, which recruited actors to the programme - a secret part of Raniere's self-help company Nxivm. Mack has since pleaded guilty to charges of sex trafficking and racketeering. She is due to be sentenced in September.
Welling played Mack's best friend Clark Kent in hit U.S. TV show "Smallville", and addressed the controversy as he spoke to another of his co-stars, Michael Rosenbaum, on his podcast "Inside Of You".
"I didn't know anything about it... I was very surprised to read anything about it," Welling said. "It's sounds very bizarre. Allison was always a nice person around me."
"It's also funny that people think that we would know, or that we would have some perspective on it."
Meanwhile Rosenbaum, who portrayed the villainous Lex Luthor in the programme, admitted he can't believe Mack would be capable of such a thing.
"And now, things have added up and... you're sort of in the middle where you're like, 'Hey, I'll always have love for Allison, but did this really happen?'," he mused. "I don't know what to say, other than if it's true, it's a tragedy."
Rosenbaum added he was aware Mack was part of "a self-help thing" when they were co-stars, although it's not clear whether or not he's referring to Nxivm.
"I remember one time, maybe it was Allison who asked me or something and she was just like, 'Hey, you should try coming to it,' and I was like, 'No. I don't know. It's not for me'," he said. "We're all figuring out what the deal is."