The outed changeling would have to flee or defect to another colony for survival. Ever since Canterlot, which was an embarrassing failure her queen had been working very hard to salvage her reputation from, ponies in general had been rather ruthless in routing out changeling infiltrations. Violently if necessary. Discovery was not an option, but she had to feed to restore her reserves and to maintain her glamour. If she had to choose between draining the swordspony and reducing his effectiveness in getting both of them through this ruin or letting her glamour fall and risk being gutted where she stood, she'd choose the former. If the illusion were to fall, there could be no other outcome for a changeling.