NewJeffCT: Immobilization doesn't grant CA (I only tell you this in case your resident rules lawyer tries to ruin your plans.)
First up, it sounds like he's probably errata'd his fighter himself. Here's how it should work:
1. Whenever he hits a foe with a non-invigorating power, he gains his con mod in temporary hitpoints.
2. Whenever he hits a foe with an invigorating power, he gains twice his con mod in temporary hitpoints.
3. Temporary hitpoints NEVER STACK.
I tried the invigorating build fighter once. Honestly, it was pretty boring because it was too easy. You might want to ask your friend if he wouldn't mind trying a different build before you start re-modifying every encounter just to give him a challenge. He might welcome the idea. On the other hand if he really loves his character as is, then go ahead and design around him.