There may be some specific cases when this is a valid issue.
I worked, for a while, for a company that worked together with a shelter for mentally unstable, and sometimes challenged, people, in order to get them back to a condition where they would be able to work again, or in some cases work at all for the first time in their life.
Those people would work for somewhat between 1 and 4 hours per day, and be assigned to an environment that was safe for them, and where they weren't able to do much damage to the company. Also, they were under near-constant supervision, but in a real company, that supervision naturally can't be 100%. So, it's not entirely impossible for someone with mental problems to get lost, enter a production area, and cause great damage to themselves, or company property.
In this case, the company had a contract with the shelter stating a) the shelter would do their best to send only people they deemed stable enough, b) limited the company's responsibility and c) have insurance to cover possible damages.
So while a guarantee like this isn't completely unheard of, the circumstances are way different. There is a greater-than-average risk of something going wrong, the guarantee is given by an organization that specializes in these things, and it's an insurance company that is going to pay, not the organization and definitely not a single individual. Also, "marketing consultant" as a job description sounds way too independent for anything like this.
So unless there are some very specific circumstances you didn't mention, i agree with the other answers: do not sign this, and recommend to the person in question to find a different opportunity.