2 things on the /index.html/
1) whenever someone made the htaccess for him, they probably put 'index.html/' in there, which is why it is displaying it like a folder,
2) the reason why it is going to the file and no showing an error:
Apache has a built in feature called "look-back"
basically it just means that if theres an url www.mysite.com/first/second/third
it will try to find the last directory or file, if it doesnt find a folder called "third" it goes back a directory and tries to find "second", if that fails then it tries to find "first"
in his case it is going all the way back untill it searches the base directory for a index.html file, which it is finding and showing.
it's mostly used to make search engine/user friendly links.
(you could even take it a step further and use htaccess to assign any files called "index" to be called as "index.html"
so his url would be www.hissite.com/index/his/stuff/that/he/typed