Hmmmm,I definitely say this is highly unprobable since in Japan they take things a little more emotional.Hugs aren't meant for "just-friends" from the time being(at least ;P).
But anyways,IF this ever happened....wow....I have some scenario about this one:
Hikari would hug him saying "I will miss you..."(in a low tone),then firstly Iris and Dento would be all like :O
Satoshi would remain static,as his mind began to get confuzed and even more,leaving him to a nervous reaction of "What should I do?",in his mind....he questioned that.The memories of past travels,the moments which they passed together and their bond would come his thoughts quickly but effectively.
The time for reactions would come after some seconds(yes,all this happened in seconds),he would hug her back...and say confidently "I will miss you too....Hikari...".
Ok,I got shippy,but anyways this situation was supposed to be unprobable,so yeah....I CAN SEE THAT IF THEY EVER PUT A BoyxGirl hug like that...