In general I have said no list A List B parties but my dd have a number of close friends so it would usually involve more than one guest. If they are mainly friends with each other but sometimes involves others then most others will understand, but if they play in a group of 4 and ds happens to see this child as his favourite of the group then I would probably say no. Whatever you do keep it subtle as children will get upset even if they have no reason to expect to stay.
I think if it is his best friend, then that is okay, as it'll be obvious to the other kids why only that child is being invited. The problem comes when a few of the party are invited for a sleepover, but not all
DS (9y) wants to have a party celebration at a venue, then once the party is finished, he wants to invite his best friend for a sleepover.
I explained that this is not a fair arrangement, as other children might feel left out. He is welcome to invite the said friend for a sleepover on a different night, but he wants it specifically on his birthday night.
I didn't want to say no straight away, so said I'll think about it.