I didn't see it so much as 'Jonathan' being in love with the girl as well - "I like her too" is him saying he likes her OK, and she isn't a major bitch or anything. I think it's more of, once his best friend's fallen in love and is getting married etc., things between both of them can't be the way they used to. He's hurt, resentful and jealous but can't afford to show it.
I didn't see it so much as 'Jonathan' being in love with the girl as well - "I like her too" is him saying he likes her OK, and she isn't a major bitch or anything. I think it's more of, once his best friend's fallen in love and is getting married etc., things between both of them can't be the way they used to. He's hurt, resentful and jealous but can't afford to show it.