The friend wasn't being accused of anything.
OP isn't accused of anything, she actually did it.
If my best friend was here, with her child. And after I noticed I couldn't find my gold necklace. Then my "pretty, clever, never lies" DD said the other child took it.
Do I a) call my best friend and ask if perhaps child has picked up my necklace, could she please check as DD seems to think she may have
b) say nothing, and call the police.
I can always do option b after a, if necessary at that point. But OP did the equivalent of jump to b. With her alleged best friend.
And can't fathom why anyone could possibly think this behaviour is not what they consider friendly, and have dropped the friendship. If someone calling themselves my best friend did that to me, I would feel utterly betrayed. It's actually got nothing to do with whether the child actually took the necklace, that's a separate issue, to be dealt with separately. It's the way the adult behaved. It's not bullying. It's saying, this is not how I want to be treated by a friend, so I'll distance myself from that person.