I have been in two different lobbies with people that have gotten S ranks, and i haven't received a chest. I also haven't received a single key fragment from any of my games played. Now i maybe miss reading how this works, but I assumed that after about 7 games that i would receive at least one fragment, or a chest for someone in my lobby getting a S rank. Any help would greatly appreciated thanks.
Not getting keys is just poor luck, they're a uncommon but not impossible to get. But as for the chest issue, I have to ask if you've already received a chest this week, because you're limited to earning 1 per week (additionally, it's once per Champion each season).
The only way for you to earn a chest by others getting an s rank is if you are partied with them.
You need to be in the PREMADE, not the lobby.
Key's are entirely random, and when not partied you have to get the S yourself, you can get one chest for each champ owned every month, but you have to get an S with them to get the chest. You're probably just a little unlucky.
