AGP>PCIe exists but no PCIe>AGP. So he's out of luck.
His new computer HAS a PCIe slot so he's looking for a an AGP to PCIe adapter, which exist, and thus he might have had been able to use it, however it does not support many cards, and as has been mentioned it's been since since announcement so it's unknown really how effective it is beyond the PR.
You are correct though the inverse is not available, although it would be technically 'possible' with the HSI chip used in the NV4x series.
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