Use Apple’s TestFlight Beta. You can send a link so your client can “redeem” the be beta app in the Testflight app or invte them to be beta testers by e-mail. However, you need OSX to upload the IPA.
I’ve got another Question if you’re willing to answer!
Say, if we wanted to make the app completely private, but permanent. is there a way we can set up a “Client Section” on the website where they can login, to get access to their archvis app, but without using the app store? or is the app store required?