Is this the bit where you're in the cavern (it's been a while since I played, so you'll have to forgive me), the girl runs off after you kill the rex's and if you move away from the middle Jimmy 'dies' even though he's not being attacked?
If so I got stuck here as well, imo it's a game killing bug but I did figure a way round it.
From the central point listen and look very very carefully ahead and right (think 90 degrees clockwise) looking at where you came in. You should see the top of a rex's head, over a ridge. Now, take pot shots and you will hit it. You'll know when you have as it'll let out a roar.
Now, it took me a few goes (ran out of ammo) but I did hit manage to hit it enough that it caused the 2nd wave of rex's to spawn.
Don't die! and it will save again.
Hope this helps and good luck.