BuddyPress.org

get_edit_html() is a method in the BP_XProfile_Field Class. You will not be able to stick it into bp-custom.php and expect it to work. That is not how methods function.

Also, this method will be deprecated in v1.1. I would recommend that you look at the v1.1 code in trunk. Otherwise, your custom coding may not function the way you expect when you upgrade.