Have that 
top that bought 
instead of wards on her first back? Is pushed all the way up to enemy turret? Buy a green ward on your first back, ward baron pit (red side) or take scuttle and ward tribush (blue side). Instant 180 seconds of protection.
P.S. I am a greedy asshole so I would never do this in anything but a ranked game.
SaltyKracka1 votesPSA: If your laners are pushed and have huge waves at the enemy turret, you can always help them by setting up a turret dive. As long as you don't suck...
Partholonian1 votesOr a countergank. The enemy jungler is going to see that wave position too.
P.S. Riven shouldn't buy under any circumstances. Her base health regen is awful and most health regen items operate as a percentage. Lifesteal is effectively her only access to decent sustain.
IcyPepper1 votesIf a Riven wants to buy instead of wards on her first back, and push all the way to the enemy turret, she better be politely asking the jungler to help her out beforehand.
Don't even bother putting the responsibility on the jungler for that one :P Vision is going to help a lot more than +50% health regen.
Honestly, two health pots and a ward would probably be a safer route if you're using your trinket efficiently.
