You'll need to have deleted as an attribute available in your index definition:
define_index 'policy_foo' do
indexes mum_policy
indexes [client.name, client.lastname1], :as => :client_name
has client_id, created_at, deleted
end
And then the following will work:
Policy.search params[:query], :include => :client, :match_mode => :boolean,
:with => {:deleted => 0, :client_id => @client.id}