Can you please elaborate?
You should not put yourself in a position where a customer owes you a lot of money. I define "a lot" as an amount I would consider legal action to recover. Charge for deliverables with clear expectations and definition of "done." If you get a sketchy or slow-pay customer who argues about your bills, drop them, they're not worth it.
Anecdotally I have known lots of freelancers in my career and slow pay/no pay is not a big problem for the freelancers who deliver what they promise and avoid the fringe customers. It's the customers who want you to clone eBay for $5,000 in a month who won't pay you. Reputable companies and organizations might take a while to cut a check but they won't screw you.
It's been a few years, but here are a couple of tips:
* Start with a warm intro from someone you've worked with in the past. This lowers the risk for both parties. This is why I suggested reaching out to former co-workers; they know you and (hopefully) will vouch for you.
* Begin with a small project first. Again, this is about lowering risk. Is there a project you can do for a couple hundred bucks? Can you write a planning document, or do an analysis of the current infrastructure with a read-only account? Or a small enhancement/bugfix?
* Document your expertise in a way that others can find you. This could be code, gists, a blog (I have multiple :) ), articles in places like dzone or the stackoverflow blog, or podcasts. This is good for finding new clients, too.
* An entirely different option is to use a contracting company, where they find the work and take a cut (a large one, typically). This way the company has trust that you can leverage. It's been decades since I walked this path, so I don't know how available it is now.
One other thing: contracting is risky for both sides, not just for the client.
For the first project or two with a new client, don't ever bill more than you can afford to lose. I've never been stiffed 100%, but one time I had to pursue a client for months to get a 4 figure paycheck. Here's that story: https://www.mooreds.com/wordpress/archives/2650
He should absolutely check his employment agreement, though, and make sure he doesn't:
* use company equipment
* do any work for a competitor
* work on company time
Contracting also is helpful because it is something he can dip his toe into without a large time commitment (as opposed to a job). I'd suggest he reach out to people he has worked with in the past that have left the company (LinkedIn is good, but Facebook might work well too). My go-to phrasing is:
"Hiya, hope you are doing well. <Anecdote, like 'remember when we shipped project X? Guess what, it's still running today'.>
I'm reaching out because I'm looking to do more contracting and to expand my network. In particular, I want to do <the type of work he wants to do>. I'd prefer to do it off hours, about <X hours a week>.
Do you know anyone who is looking for a hand? If you do, or think of anyone, I'd sure appreciate an intro."
This phrasing is far less direct than "Do you need help" but conveys the same message.
He should also think about a rate that he'd work for, as that will likely be one of the first questions any possible client will have.