If it's a paid app in AppWorld, you can use the "gift" option to buy it and send it to them.
If it's an app that you wrote for phones: Just upload it somewhere and send them the url (signed, if necessary).
If it's an app that you wrote for the PlayBook: make sure it's signed, have them put their tablet into developer mode, and use on of the many tools available to send it over the local network.