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Strikes me they're both at fault:
1. Jon is superficial;
2. Kendra is superficial
OMG they're both superficial. Lock 'em in a room together and see who comes out alive.
I say tell each of them separately that how Jon talked to Kendra was very rude, but for your own reasons you choose to be friends with each of them. State that you are not a mediator who is assigned to resolve their conflict, they are old enough to do this on their own.
Then let them make their own minds up.
Or if you do chose to make a choice and drop one of both of them, you might want to let each one know from you directly (rather than the gossip mill) what you decided and why.
Gentle hugs, hope you resolve this
You have to do what best for you and if they don't like it then I guess they can both go. Don't make yourself hurt for others.