You'll have to break the external reference. Create a copy of the file and break references on that. This would save the original file references intact.
I've not tested with locking references though.
If you just want the part in its current form, without any of the reference parts/assemblies you can break all the references. You may want to start by saving a separate copy of the file first - just so you keep it separate.
Selecting Break all will permanently remove all the links. When checking for references after this point, they will all be shown as broken(when list broken references is applied). This should allow you to send the part to your client who can then open it without trouble.
As stated earlier, I would still advise to create a copy and break external references in that. Because references once broken can't be fixed.
If the customer does not need to see the feature tree, save the part as a Parasolid or STEP or IGES format, and send that.