It's not really THAT bad, we all learn by doing.
Go to BI's "settings" => "Users" tab, click on the "Help" button and read up. BI has excellent and contextual built-in Help.
You'll discover how to create a "Group", assign a camera or cameras to that group and then allow that user to view only that group, thereby allowing that user to see only the cam or cams you want them to see. Be sure to un-check "Administrator" privileges.
You could probably stream it out of Blue Iris to give them just access to the feed, but not ability to search. Maybe on a restricted channel on YouTube?
You could also allow access only to the IP of your BI machine and the port BI is running on in your router. That should keep them from being able to access everything else.
For clarity: Yes, you can limit them to the cameras of your choosing within the BI server itself. However, as @Mike A. pointed out, they will have access to the other systems/services on the local area network (LAN) that the BI server is on.
If the BI server is on your office LAN, along with network printers, files servers, access control systems, etc. Anyone connecting via the VPN might also have access to the same devices unless you take precautions to prevent it. If your IP cameras (network devices) are not isolated to their own separate network (connected to BI server by a 2nd NIC) they (VPN visitors) could potentially connect directly to the camera GUI.
You could do both - have your VPN and put their cam on a forwarded port, DMZ, etc. But then their cam would be sitting accessible to the Internet and a gateway into yours which isn't a real great idea.
With a more advanced firewall you likely could restrict VPN client access better but that's probably beyond what you have there. I don't think that there's any way to do that simply using a typical home router/client.
ETA: I guess you also could set up another VPN using a Raspberry Pi or whatever on a separate subnet/VLAN and give them access that way. You'd need to set up the BI server on both. That's getting a little more complicated though. Might be able to use the same front-end VPN server with different client subnet/IP settings but then they could change those easily.