I've won games with Rengar against riven, but its really hard. It only takes one kill for Rengar to snowball off riven and same applies with riven.
You may have to rely on your jungler to help you and keep riven near your tower, since shes bound to push if she wants to be aggressive with you. I constantly harass with E and Q if she engages on me, you win her in sustain but she beats you in all-in damage. You should try and minimize the damage you take so you don't have to use your empowered W to heal. Thats the key to be honest, constantly kite her with your E and rely on your jungler and play safe.