I'm not sure how the medical insurance work in your company, but usually most companies provide medical reimbursement to their employees.

You sited an example: sewing at home for the company of your friend? let's say, God forbid, you got injured and is unable to report for work, more or less you will take a sick leave from your work or you need to seek medical treatment from the hospital/doctors. And you are entitled for medical insurance from your company/sponsor.

Then, who you think is responsible to pay for your sick leave? Which company you'll reimburse for your medical expenses? From your friend's company or from your sponsored company? Definitely from your sponsored company right?
Though the injuries was due to some work with your friend's company.

That's mostly the case:)

:)