yep, rabbits chinning something is more often than not just there way of letting other rabbits know that this area or thing belongs to them, and like a lot of animals out in nature that mark their territory it is just a way of letting themselves and other animals know that this is now home ground. Just because your bun doesn't chin you, doesn't mean they dislike. It just means they recognise you already as familiar and part of their home and life. My Bun has never chinned me, because her scent is already on my clothes and hands and my voice and presence is familiar to her, so she doesn't need to, but anything new I put in her pen, she will always chin it.
So your bun obviously recognises ur friend and is comfortable, but now your Bun is just saying you are mine and in my territory. But your Bun already knows you and loves you and can already smell its scent on you, so it doesn't need to claim you. So don't be too stressed and worried about the chinning.
Like artistchibi said, Buns can also be curious about things that don't have their scent, but are comfortable with, and when they realize it doesn't have their scent they will chin it. I know a Bun once that chinned everything in their pen, they just went around sniffing everything moveing things, and chinning, and they didn't do this again for another few months, and then they repeated this action all over again. Sometimes Buns will just do this if they think their scent isn't strong enough. Its just a natural territory action they take.