It's a great use of Leanpub. We're glad to help, but the good news is that we don't need to do anything special here. The following process will work for you:
Create a new Leanpub account & book for your father. You can use his email address for this.
Presumably he does not have a Dropbox account. Assuming this is true, when his email address gets the Dropbox sharing request from Leanpub, have him forward the email to you. You can either:a) just accept the sharing request using your Dropbox accountb) accept the sharing request by creating a new Dropbox account for him, then send your Dropbox account a sharing request on his Dropbox folder
Regardless of how you did #2, you're good to go. You can edit his book on your computer using your Dropbox account. Whenever you want to publish a new version for him, just login as him and click publish.
Note that the book will sync with the Dropbox account that accepted the sharing request in #2, so you will see the generated books a bit faster if you just use your Dropbox account, since there won't be an extra Dropbox sync step through an intermediate Dropbox account.
If this doesn't make sense, please email firstname.lastname@example.org and we'll be glad to help.