As long as the described sequence of events was fluid and didn't include any significant pauses, I would not intervene.
As I see it, the players have not yet committed an illegal action, since the rules have provisions for rewinding spells when you cannot comply with all the steps of casting them (in this case, the player playing the
Force of Will couldn't finish paying its costs). He wasn't aware of the Trinisphere effect when he started to cast the spell, but his opponent reminded him of it and then realized the action was not possible.
On the other hand, if there was a significant pause after the playing of the FoW and the subsequent actions, I would consider GRV for an illegally cast spell (and maybe FtMGS depending on the details of the situation). Even more so if they actually resolved the counter.