I suspect that it was OK for them to check since his mum invited them upstairs...
I guess I/we am/are not as trusting, since our police are heavily armed. We need them to be armed to uphold our rather harsh laws, but a whole new, additional set of responsibilities goes along with that acceptance. They can not expect to perform a search of a premises with no legal probable cause to suspect a crime has been committed.
I can't even imagine that they would show up at a citizen's door and expect to be politely invited in to investigate a reported crime (fourth and fifth amendments to our constitution), with absolutely no proof of any crime having been committed.
I live in a dream world, I suppose.
I'm an American.
Of course, once she allowed them access to her home, they were able to extend the scope of their search at will to include multiple searches, FFS!.