You do need to have a discussion with her about stray threads. I go over customer's quilts many times before it's folded and returned. Heck...even the process of folding a quilt will show both sides.
If you're finding tangles in the actually stitching, that's not right! There should not be any excess thread in the actual stitching.
Do you look at the quilt when you pick it up? That's a good time to start the discussion. See what she has to say. Then when she calls you to pick up the next one ask her if she's trimmed all the stray threads.
Good luck
Nan- Indiana