You need a developer certificate to put an app on any iOS device with a stock OS.
What kind of certificate and how many you need depends on the type and amount of distribution you require, and the size of your client's company.
You most likely will need to join the iOS developer program ($99/annum) yourself to install and test your apps as you develop them. In addition you can deploy Ad Hoc installations for up to 100 devices (including your own, your testers, your clients, plus, very importantly, including all repair replacements and upgrade devices).
Your client may not need any license if they only want a few copies and are willing to have you renew your Ad Hoc installs a few times per year.
If your client wishes to deploy an app in their own name outside their company or through the iTunes App store, then they need to apply to the iOS developer program themselves.
If your client is large enough to have a published Dunn & Bradstreet rating, and wishes to deploy only to their employees, then they can apply to the more expensive Enterprise iOS Developer program, and run their own internal app store.