For one, he can transfer that expensively managed RRSP to a lower cost RRSP. RBC's action direct comes to mind, and they're in the US banking system too. (Many Canadian banks use RBC as their clearing house.) If he leaves it in a (low cost) RRSP, it'll still continue to accumulate pre-tax. He will want to carefully time his eventual withdrawals to minimize tax.
If he took it out of the RRSP yes, he would pay tax on the withdrawal immediately. There would be a withholding on the Canadian side, which he could file taxes to CRA to reduce, and he would have to declare the income to the IRS, who will be happy to collect their piece. And It's unlikely he'd have tax-advantaged accounts available to him right away in the US if all his recent income has been Canadian sourced..
I'm assuming if he's ever opened a TFSA he's aware of the restrictions US citizens should follow to avoid the passive trust trap. TFSAs have never been folded in to the tax treaties.
Pkmaine provided the tax treaty link, but in short, your friend is likely best off where he had the longest working history. I will eventually collect CPP, my spouse SSN. You can collect either in either country. An interesting side note is that CPP is fully funded on a going concern basis, while SSN holds "trust fund debt" from the federal government. Might make a difference in those sustainability discussions.
I look at our retirement fund savings on both sides of the border as an extension of diversification. Although we do have to watch out for denting that advantage when we've carelessly invest in overlapping assets on both sides.
