Once the client registered a free account on Divhunt, you can add him as collaborator on your specific project which you want.
Then using the login link, which will be provided to you, you can share that link with him, and he will be able to login and manage that project specifically.
In this way, the subscription is kept on your account, and you have the ability to manually on your side charge the client for hosting as you wish without him knowing the real price of Divhunt (in your case it’s lifetime).
Another option will be transferring the project to your client, in this case you get your subscription back, but client’s needs to purchase another one.