You most certainly must let the guest know. Otherwise they will very likely demand a refund for the entire stay, with good reason. Have seen examples of this before.
How about you either reschedule the visit by landlord to a vacant time, or give the inspection day free to the guest?
You should just explain to the guest that you will be leaving the property soon and your landlord has the right to have people view the property. You also usually have the right to define what that means (ie no one turning up at 8am in the morning for example when you are still in bed). Agree a time when your guests will be out of the property - these appointments are usually made fairly last minute, so it should be possible to keep your guests happy and your landlord.
I certainly think the guest would be within their right to cancel their booking. I am sure there are a lot of guests who would be pretty unhappy with this situation. I think the best you can do is try to get your landlord to commit to their dates in advance and communicate clearly to your guests. I would also consider offering them a free night as compensation.