Uh, you gotta be kidding me. Ask anyone with bladder/urinary problems what happens in a day - extreme pain, bloating in the lower pelvis if they can't void the bladder. Then, in 2-3-4 days, it can cause kidney disease, require dialysis to treat, and can be fatal.

"Acute urinary retention, a potentially life-threatening medical condition, requires immediate emergency treatment. Acute urinary retention can cause great discomfort or pain."

Urinary Retention - National Kidney and Urologic Diseases Information Clearinghouse

"Acute urinary retention can cause urine to overflow back into your kidneys, causing permanent kidney damage complication.
Both acute and chronic urinary retention can lead to kidney disease or failure, that may require dialysis or a transplant just to keep you alive. So, always share your “can’t pee” symptom with your health care provider."

Cause of Acute and Chronic Urinary Retention, Urine Retention, Inability to Urinate Symptoms ? Health Blurbs

I realize your friend thinks it's voluntary, and the debate is about people with no inability, but the result, IF someone were capable of holding it that long, which is extremely unlikely, would be the same as for someone who couldn't urinate.