The slap was spontaneous, so not planned and therefore not a brilliant plan by Maya. What kind of ideas do some people get?
But the girls who saw it note that Quinn did nothing about it, or that there was no serious reaction from Quinn. That makes Quinn a toothless tiger and therefore vulnerable. It has weakened Quinn, and when there is blood in the water, the sharks come. It's just a matter of time.
Mona and Riona have definitely seen Maya beat Quinn. Mona leaves after probably talking to Stephen Burke. But what they discussed, we don't know. Riona is slowly but surely turning away from Quinn, as we can see in EP8. Sage has also heard about the incident and is talking to Maya and trying to get in touch with Mona. Camila is also not as attached to Quinn as she was until the end of EP6.
Yes, Lily has gotten a little closer to Quinn, but that doesn't mean anything because Lily wants drugs and Lily herself has sharp teeth that she can use to inflict gaping wounds on Quinn. Lily has shown from the beginning that she is not impressed with Quinn.
Because Sage feels connected to Maya, because Sage was the way Maya currently is, Quinn is further cornered.
Can we now assume that the slap was the catalyst for all of this? No. But the slap hastened Quinn's likely demise, and it did so without an elaborate plan.