google doesn't have a magic percentage threshold-that's not how their algorithms work. The real question is whether the copied bits are generic connective tissue or core arguments someone lifted wholesale.
If it's just standard industry phrasing, you're fine. But when whole sections read like a lightly paraphrased Wikipedia entry, that's a signal the writer hasn't added their own thinking. I'd send it back and ask for genuine value, not a remix. Originality isn't a number-it's whether you're bringing a fresh slant to the table.