Can not so fast be used on Riposte?

No, you cannot use Not So Fast to counter Riposte.

According to the RiftJudge FAQ (ID 8559):

"Ruling: No, you cannot use Not So Fast to counter Riposte. Not So Fast can only counter spells or abilities that target units or gear friendly to you (the player of Not So Fast). Riposte targets a unit friendly to its own controller, not your units, so it does not meet the targeting requirement for Not So Fast."

Reasoning:

• Targeting Requirement: Not So Fast specifically requires that the spell or ability it counters must be choosing a unit or gear that is friendly to you (the player casting Not So Fast).
• Riposte's Targeting: Riposte requires its controller to choose a unit friendly to them. Because Riposte does not target your units, it fails to meet the necessary condition for Not So Fast to be played against it.