The TV favourite, 57, posted about his mate Paul Matson - a formerly homeless armed forces veteran who now works with a group called Hull 4 Heroes.
Nick shared a clip from a podcast interview with Paul, in which he discusses being "destitute" and weighing only "six and a half stone" when he had nothing to eat.
What happened to Nick's friend Paul?
"I was sat on my doorstep, basically in the gutter, if you like," he says in the snippet, taken from Blue Collar Conversations.
"Feet off the curb. Destitute," Paul continued. "I was six and a half stone. Didn't have anything to eat, didn't want to beg. I just knew, I didn't want to do it any more, didn't want life any more."
In the caption of his Instagram post, Nick asked his fans to "work together to make the world a better place".
He wrote: "This is my friend Paul Matson speaking about his time as a homeless veteran.
Couldn't be more proud of my friend.
"He is now at the heart of organising support in the lockdown for carers, NHS, emergency workers, veterans and Homeless in Hull through @hull4heroes who have an ambitious plan for a veterans village.
"Couldn't be more proud of my friend. Time for us all to work together to make the world a better place, not just for veterans but for everyone."
Followers found it heartbreaking
In the comments, one of Nick's fans said: "My heart goes out to anyone living in the streets. But especially our veterans, who we expect to step up and keep us all safe."
Another wrote: "Heartbreaking. Well done for helping. You're an inspiration."
Someone else said: "This is dreadful, should not have happened to this brave man."
Nick stars in DIY SOS, which had viewers in bits last week (Thursday, May 14).
The episode, a repeat, tasked a team of volunteers with building a new home for veteran Simon - a single dad who had his foot blown off in the Iraq War.
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