[CM30]

Depends exactly what you need. If the site only needs to provide a bit of info about the business plus a contact form, then just about any CMS or site builder would be perfect here. WordPress could do this, SquareSpace or Wix could do this, and heck, you could even do it manually just by buying/finding a template (maybe a Bootstrap one?) and coding a few static pages. As long as you don't do anything obviously disastrous for SEO (like say, making it entirely reliant on a poorly coded SPA, sticking your text in images, not including title tags, etc), it should work perfectly fine regardless of what you do.

You'd probably want to choose the CMS or site builder if your plumber friend wants to actually maintain the site and add content though.