The US reality star was housemates with Steph and Jez when they fell in love on the show earlier this year, despite Stephanie being in a relationship with model Sam at the time.
After leaving the house, she confirmed her split from Sam and began dating Jeremy, but it all fell apart this week when he admitted sleeping with another woman after his eviction while Stephanie was still inside.
Now, speaking to The Mirror, his co-star Tiffany has said: "[Stephanie] should have stayed with her boyfriend. I know that she is kicking herself in the ass over that because look how Jeremy did her in what... three weeks? And he's already sleeping with other women.
"I feel bad for Stephanie but at the same time, I feel like she should have been smart enough to see this coming. These boys don't want you! Getting with Jeremy while she was in a relationship was a bad move. She made a horrible mistake.
"I never thought that whole Stephanie and Jeremy thing would last."
Outspoken Tiffany also said Stephanie didn't deserve to have Sam back.
A rep for Stephanie told The Mirror: "We don't need to give this a response."
It's been a difficult week for former Hollyoaks actress Stephanie, 22, who sparked concern among fans when she tweeted and deleted several messages showing screengrabs of private conversations between her and a C.Pope, believed to be club dancer Caroline Pope.
There had been rumours earlier this year that Jeremy had slept with Caroline in Newcastle after his eviction, while Stephanie was still in CBB, but both Jez and Caroline denied it.
Yesterday, however, Jeremy, 25, confirmed it had happened.
He told Irish TV show Today: "I was with someone that night. It's true. I didn't deny it, it just happened at the time. Stephanie had her own issues [with Sam]. We were both single... Since I've been with Stephanie I've never hurt her. I didn't know [once she left the house] if she was going to get back with Sam again."
Stephanie has yet to respond to Jeremy's interview.