I'm with
@mudcat on this one. If you don't feel comfortable with it, I wouldn't do it.
I only have one friend I would do this for, and it wouldn't bother me to do it, as I have the experience. But we've known each other 50 years, she's closer to me than my sister, and even though she doesn't sew, she trusts my judgement regarding sewing implicitly as I trust her on any engineering matters. We truly know and understand each others strengths and weaknesses.
With any other friends, I could see the ability for something to go wrong and end the friendship.
Plus once you cross the "could you just?....." line - it gets harder and harder to say, no.