Whether you are inquiring for a "friend" or somebody else (like yourself), you need to do a "market research" before launching a business.
1. There is a world of difference between being a plumber in a company and becoming an independent plumber. As an employee, you have a safety net under you, but as a contractor there is none.
2. Look up at local plumbing services advertisers - yellow book, paper ads, on line, leaflets and all other sources - and you tell me: how's the competition? Plumbing is probably the one trade with the most competition.
3. If you want to do re-pipes and re-pipes only, and you manage to get one job a month, you won't make money.
4. If you want to establish yourself based on "quality work at affordable prices", you'll have to do many jobs with a very small profits - it might take longer than you think to have the phone ringing based on words of mouth.
5. A much better field in plumbing is converting oil furnaces to gas furnaces, IMHO.
So you see, you don't need to go to Wharton School of Business to know the risks of becoming an independent contractor. Good luck to you.
A friendly advice: don't mock other trade contractors' quality of work , if you don't other contractors mocking yours (contractors like drywallers for example).