Even if you don't charge for it screwing things up may damage reputation of your customers.
His reputation actually can be damaged if he did something wrong. I'm not against with someone, newbie, jr. member or any rank that wants to work as a campaign manager, you're all free to do as long as you are complying to the forum rules, obeying and being a model to your participants since you are representing a company.
Just don't let yourself involved in something sketchy and shady projects / companies.
If someone hires to you be their campaign manager, make sure that you'll escrow funds with a trustworthy and reputable person in the forum to secure the rest since you are offering it for free.