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The LAMP service is intended for Cornell departments so they are set up to require a NetID. That pretty much leaves you with two options.
You could sponsor a NetID for the outside developer, there is more overhead initially with this option, but in the end the outside developer would have free reign without any intervention from you.
The information on how to do that is located here:
http://www.cit.cornell.edu/identity/netid.html#who
I think you'd be interested in the "Exceptions with sponsor" section. Also, it might be a good idea to understand some of the responsibilities that go along with sponsoring:
http://www.cit.cornell.edu/policy/interim/AuthenticationITR.html
Another option is your outside developer could do all their work, create the content, etc. and send it to you. Then you could use your own LAMP account/NetID to do the file transfer to the LAMP server. There's less to do initially because you already have a NetID and LAMP account. But in the long run there is more involvement and overhead for you as content changes are made and so forth.