Actually, I think it may just be Font of Radiance that's the issue with this build. All the other crit-dependent stuff isn't too bad. Font of Radiance was a marginal feat when first released in PHB I, but since then there have been a lot more radiant effects released and it's incrementing upward. (I'm surprised nobody has yet brought up Rod of the Dragonborn + Radiant Breath from the recent Bahamut article. Picture a Frostcheese Dragonborn Sorcerer all of whose spells are Radiant+Cold... or, probably less effeciently but more entertainingly, a Dragonsouled Revenant whose spells are all Necrotic+Radiant+Cold.)
Prior to Font of Radiance, it's not nearly as spectacular; IMO it's just that one feat which is a potential problem. And the OP implies that the PCs are not yet even Paragon tier. Can I ask what level span you're honestly expecting for this campaign? 'Cause that's a long time out before you have to worry about that element of it... by which point a whole lot of water will have gone under the bridge, and a whole lot of sourcebooks been added to the universe, and the build may no longer even be of interest to you.
So, frankly, as a DM I would personally allow the character but let him know that I may end up somewhat nerfing Font of Radiance. (For example, it would still be a good feat with "until end of your next turn" instead of "(save ends)" on the effect. At which point it's +3-9 d6, on a critical hit, with this at-will, at epic. Nice, but meh.)
Also note that two elements of the scariness of the build actually counteract each other... Hand of Radiance can hit targets on opposite sides of a room, but the Font of Radiance additional damage hits things adjacent to the critted target. If you have to use positioning control to group your monsters together to get the full benefit, then it's no more powerful than Blazing Starfall at that point. In fact, since you'd really have to use positioning control ahead of time to make best use of it, but you don't know which target (if any) you will crit (barring MC Sorceror, anyway)... I'd say that the versatility of Hand of Radiance is already being made less and less useful by using it in this combo. (Or, alternately put, the "+3d6 to adjacent enemies" is less likely to trigger if you make proper use of Hand.)
As a more interesting fix, and one I may consider in my game if our Invoker gets excessively powerful compared to the rest of the group (not likely, doubly so since this is her first time gaming), would be to anti-burst the power. "Three targets, none of whom are adjacent to any other" or even "Three targets, each of whom is at least three squares away from any other." Now that's a cool and interesting tactical setup!