Sharing Photo Stream with Windows user

Greetings. I am trying to help a family member join Shared Photo Streams on their Windows PC so they can see the photos of their grandkids. We installed iCloud Control Panel 3.1 on his Windows 7 notebook, and I created an iCloud account for him using my MacBook. We can log into the account on iCloud.com, so we know it has been set up. We checked the Photos boxes in iCloud Control Panel.

I added him to an existing Shared Photo Stream using iPhoto on my MacBook, and he received the invitation via email, but when he clicks on the link to join, all he gets is a page that says you can only join a Shared Photo Stream using one of a list of Apple devices. I have read all the troubleshooting posts, and we even tried the hint that suggests killing all the Windows proceeses and restarting, but no joy. I'm successfully sharing the stream with other Apple users, so the basic sharing is working.

I know we could set up the Photo Streams as public Web pages, but we'd really prefer not to have the baby pictures available to anyone who happens to find the URL. Some posts I've read seem to indicate that other people have this working, but we just can't figure it out. Any help from someone who has this working with the software combo above would be greatly appreciated. Thanks.