MI5 only have a supporting role to the police forces in addition to their "secret" stuff. They wouldn't vist your friend. If there was something he did he would've been visited and questioned by the police.
Chances are he was drunk and brought home some random guy from the street and couldn't explain what happend in the morning after, so he invented to put his mind at rest.
Yeah, I know, that last paragraph was stupid. I shouldn't have written it. But then, you shouldn't have been bu||sh!ttin.