Absolutely! The US National Arboretum maintains a list of what's in bloom during different times of the year, which might be useful to your friend.

Here is a link that might be useful: What's Blooming

Thank you for the link. The page gives what's in bloom at each time of the year in the US, but it doesn't give specific locations. On the page it says, that the locations are charted elsewhere in the "ArboretumÂs Visitor Guide" but I can't find it on the site.

Actually, it shows what's in bloom at each time of the year at the US National Arboretum, which is right smack in the middle of DC. Their address is 3501 New York Avenue, NE
Washington, DC 20002-1958. The locations they're talking about are the locations within the arboretum itself where you can find the different plants in bloom.

I figured that the list should be more than sufficient for your friend to get an idea of what is in bloom during the fall months of the year. It's well worth a visit if you haven't been.

Suja