The 40-year-old circled the car four times while scraping a key along the paintwork.
She had just been told by a private detective that Michael had spent the night with 31-year-old Sam Bunn - whom Michelle had considered a friend.
“Seeing pictures of them together was the worst moment of my life,” the Sun quoted her as telling a friend.
Michelle and Michael, 46, launched Ultimo from a spare bedroom and built it into a 45 million pound empire. They announced their split two days after Christmas - both insisting no one else was involved.
“He went off to a friend’s house for the rest of the day. Michelle ended up eating it in the kitchen with her and the kids in tears. It was heartbreaking,” the friend said.
Michelle suspected Michael of having a relationship with Bunns - a designer she trained up and even went jogging with - but he always denied the claims.
She hired a private investigator when her husband, Bunn and the other staff went on a trip to the offices in Hong Kong and China. The detective told her that the two had disappeared off to a room together in the early hours when others were partying and showed her pictures of them together with their arms around each other.
Michelle then threw Bunn’s clothes out of the Glasgow house she had loaned her, changed the locks and sacked her before keying her husband’s Porsche Panamera. The four door cars cost up to 120,000 pounds.
Michael has denied cheating on his wife before the rift.
“Prior to our split, there was no relationship,” he said.