Boris is a diplomatic incident due to a gaffe waiting to happen, imo. He's simply bound to say something stupid again - he has form. His "Boris is a lovable buffoon" schtick can play ok to many in the UK, but he'll not receive such a forgiving audience abroad, where equally powerful politicians...
Boris as foreign secretary? Well, f*ck me. Best not let him near any foreign kids if he's carrying a rugby ball. Best not let him talk about the Hun. In fact, in the interests of time, it's easier just to say "best keep him away from any actual, err, foreigners".