Avalon Single Supplement Policy

[happycruizer]

We are checking on a river cruise on Avalon and a good friend would really like to join us. However his DW will not fly overseas nor does she want to leave her family for an extended period. I know the cost for a single is considerably more expensive and Avalon says they have to "request" a "single" as all of their cabins are for doubles. Has anyone had any experience with this? The sailing we are looking at has very limited availability, so would Avalon refuse a single in favor of booking a double? Will they hold a cabin until approval of a single is rec'd? I understand the fact that they'de rather sell a double cabin but with the surcharge for a single they won't be losing that much.