Can a colonoscopy be performed if there is a urinary tract infection?

Yes. If there is a treated infection, she can still have the colonoscopy.

Although a urinary tract infection should not interfere with her colonoscopy, this would depend on how severe her infection is and how she feels. Simple UTI's can be treated within 3 days. It is usually recommended to wait after the infection is treated unless she is asymptomatic.

As long as she can tolerate drink the preparation, she should tolerate the colonoscopy just fine. She should contact her gastroenterologist to clarify.