Hey Fellow Minnesotan!
Now that I've looked up what this item is (like @Sarah977 ) I'm in agreement @Bryan186
that it should NOT be left out if your friend is hosting guests. Despite it being a hygiene tool, guests often complain about innocuous items (hair in the bathroom, bugs) and request refunds or rebooking from Airbnb. Plus, I'm pretty sure most female hosts don't leave their hygiene products out in shared bathrooms. I can imagine what would happen if this were discovered by guests!
Also, depending on the guest(s), some might play/tamper with the device. Others with a conservative disposition or traveling with kids may not appreciate having to confront this item.
In searching for information about this device, I also saw a post where a gay man asked how he should deal with this matter since he has a straight roommate. Suggestions pushed for him to "switch adapters" or use a diverter so it could be removed.
If your friend chooses to leave the device in the shared bathroom, then he should definitely advise guests about it at some point so that they are aware of its existence and can make a decision prior to confirming the booking.
Should this be in a host's personal (private) bathroom, you would have a valid point about leaving it out. However, based on how you describe the setup, I forsee more problems than benefits for him as a host.