Barlow, 43, is alleged to have invested in two separate tax avoidance schemes, variously used by his bandmates and other celebrities including George Michael, Arctic Monkeys and Sir Michael Caine.
He was ordered to pay back the money by Prime Minister David Cameron in Parliament, and faced calls to be stripped of his OBE when details of the first scheme, Icebreaker, were revealed in May, reported Digital Spy.
Collins said that he supported Barlow throughout the difficult period, and argued that the star had been subjected to "horrible" and "ugly" abuse after being made a scapegoat.
"I notice it more with Gary than anyone else, that as soon as something comes up where people feel they can discredit him, they come down on him like a ton of bricks," Collins said.
"I don't know why people are so quick to wish people to fail. It's such an ugly trait.
"I don't sit there and go over Gary's financial details, but I do know that it wasn't just his name that got brought up. I didn't think that was fair, it's horrible."