Thank you for your very helpful comments and link.
I’ll try to find out if there have been any more changes since the article in the link is now 5 years old.
Re the tax on a future house sale if he lives in it for 2 years: I assume he could elect to include at least $25,000. in capital gain as investment income in line 4g of Form 4952 (Net Investment Interest Expense). Could he do this also with the added proration adjustment to his gain and/or with the unrecaptured depreciation of $17,400.?