Can I visit a hotel lobby to visit the receptionist? [closed]

Yes. You don't need a reservation to do a walk-in so clearly reservations are not a pre-requisite for entering the lobby. You also don't need a reservation if you're at the hotel for a conference or if you're going to dine at a restaurant inside the hotel that's accessed through the lobby, etc.

I don't imagine a short visit would be a huge problem for most hotels. Much of the job of receptionist is about talking with people. Of course you shouldn't distract your friend from their job, nor make it a regular thing.