Rails link_to syntax in order to add some inner html
You just skip the first parameter, can wrap the rest in parens, and then add the do/end.
<%= link_to(user_user_relationship_path(id:@relationship.id), remote: true, id: "follow_#{@user.id}", class:"btn btn-small btn-danger", method: :delete) do %> <!-- your button html here --><% end %>