Not my kind of sewing project - I would use duct tape also.

You guys are funny..... duct tape is definqtely the way to go!!!!

I agree with duct tape..amazing stuff!!!

I take tarps and make them into BBQ covers regularly. They last, waterproof for years.
I heat seal them before and after sewing with a cool iron. Sometimes you have to play with it on scraps till you get the right temp.
Sewing needle holes really don't make a difference as far as I have seen.
If you want to do this for your friend, it will buy her a few years on a replacement.
MaryKatherine

How huge? How heavy? How expensive to replace?

Put some wide tape over the seam and sew with a large stitch either side of the seam so that you don't perforate the plastic and make it easy to rip out accidentally.

I think it would be a waste of your time...I agree that duct tape would do a better job...

I have re-seamed heavy duty plastic tarps - but I do a double seam -like your jeans - and have never had a problem with them leaking. My brother and SIL are using one I repaired a few yrs ago on the back of their house -they have torn off the back 1/2 to make a bigger bath -laundry room and sewing/craft rooms and its rained every other day since they started and the rest of the house has stayed dry.And I did not use Duct tape on the new seam..but I used fine fishing string instead of thread.

Duct tape or use for smaller cover ups. If you sew it, it will only create a perforation that will make it easier to tear in the future. Tarp isn't that expensive. The only other idea would be to put the 2 pieces together and fuse them with an Iron. You put the 2 pieces together and use brown paper bags/shipping wrap under the tarp to protect the surface you're working on and on top of the tarp between your iron and the brown paper bag/shipping wrap. Worth a shot.

I don't do that type of repairs or sewing. I would use the smaller pieces as tarps and just buy another big tarp. Plastic sewing probably wouldn't work.