Getting canvas-work aka tapestry stretched

If your friend had used specialist tapestry canvas it shouldn't have distorted. As that type of canvas is very stiff . Is she going to make them into cushion covers if so is she going to see a pre zipped cushion back. Framers have specialist equipment they use to pull linen and Aida fabric back straight before framing. Perhaps they could straighten it for her.

Ask at craft shops who sell tapestry items, they may know someone. You can do it yourself - there will be info online but I’ve never managed to get it as good as the professionals do.