I think the Web way is the best approach… but a WebService shall do the tricks also.
I suggest to :
1) build a special view that will include only data that you want to share (for example : client number, client name, client status)
2) create an ACL entry “Anonymous” with reader access (and set the maximum internet acces level to “Reader” also)
3) Give the URL (with IP adress, if it is not “secret”) to access this view to third party
(ie => http://18.104.22.168/DatabaseReplicaID/ViewReplicaID?OpenView)
NOTA 1 :
If you didn’t have already a web access to your Notes application, you have to load the “HTTP” task on the server.
Iif you want to restrict even more the access to this web view, you will have to work with an other Adress Book to authentify the third party user.
Is it enough to start ?