I wouldn't be quite so definite about him having typed these search terms. I would agree that Google thinks he did but there are at least five other possibilities.
Firstly there is some malware which deliberately adds items to your search history, presumably in an attempt to encourage you to visit the websites concerned.
Secondly it is possible that some software has been conducting Google searches on his behalf. So, for example, if he visits a porn site it could pop up a screen with a Google search for "freesexdates". That would then appear in his search history although he himself has never searched for it.
Thirdly if he signs in to Google (as many people do these days) it is possible his account has been hacked, probably in an attempt to get access to his email account and use it for spam - this happened to me. If his account has been hacked any searches conducted by the hacker will appear in his search history.
Fourthly there was at least one instance earlier this year where a bug in some third party software meant that Google got confused and recorded searches against the wrong user.
Finally there are so many threads on Google's support forum from people complaining that their search history shows searches they haven't made that I wonder if, despite Google's denials, there are bugs in their software which can cause this to happen. It clearly doesn't affect everyone but it seems over 18,000 people have experienced this problem.