As revealed at the weekend, the pair team up against Phil (Steve McFadden) in next week's episodes - determined to get Roxy's money back by stealing from the safe in his house.
However, when Phil catches the duo in the act and an argument follows over Roxy's missing cash, the Albert Square hardman protests his innocence and insists that Glenda was the thief. Later, when a stash of money is found in Glenda's wardrobe, Roxy (Rita Simons) believes her cousin's lies.
Speaking to Inside Soap, Barber explained: "Glenda doesn't think that Roxy will take Phil's word over hers, so she's devastated when she does. It makes no sense - Glenda is broke, so why would she leave a pile of money in her bedroom cupboard?
"She instantly knows that Phil planted the cash there. Glenda's so hurt that Roxy believes him instead of her."
Glenda's heartbreak leads her to reveal the truth about her affair with Phil in front of Shirley and a number of other Walford residents in the Queen Vic. The scenes air on Thursday, March 3.
> Click here to read our EastEnders spoilers