Oh yes, they seem to have changed the structure of the URL for lost passwords in 1.5.

Shame nobody noticed before 1.2.1 of nBill was released!
You can change it in /components/com_netinvoice/netinvoice.html.php, line 111 (ie. replace "index.php?option=com_registration&task=lostPassword" with "index.php?option=com_user&view=reset") and also in /components/com_netinvoice/html/orders.html.php on line 170.
I will fix it for the next release though.