Hi all,
I'm actually using the drupal 6 version but see that this still applies in 7. The function user_relationships_ui_actions_between doesn't pass it's text through a themeing function so it's can't be overridden very easily. This is specific to 'requested' and 'received' function calls. The text:

You have sent a new %relationship_name request to this user. (!pending_requests)
and,
This user has requested to be your %relationship_name. (!pending_requests)

is hard coded in the actual function and then passed to theme_item_list. I tried to change the output by also calling hook_user (hook_user_view in d7) to change the text but because it uses the place holders for relationship name and pending_request, it wasn't as easy.