She couldn't bring herself to tell him directly where he needed to improve, so she told him in a gentle way.
「やんわり伝える」とは英語で「to soft-pedal it」を言えるでしょう。「she soft-pedaled it」(pedalはピアノのペダルのこと。あるペダルを踏んだらピアノの音が静かになります。
単刀直入 = straight to the point, point-blank, frank, direct