How do you block porn from appearing on your Mac?

[Claire Green]

I need to block all porn content from my Mac. My young friend uses the computer and I notice that when she is logging out, she clicks out of porn sites that are opened and tells me she doesn't know how they got there. The question I have is can porn sites pop out of nowhere and show their content out of the blue? Should I get the internet filter for Mac to block unwanted sites? Please help and excuse my ignorance in this. Thank you.

[The 8th Immortal]

My young friend uses the computer and I notice that when she is logging out, she clicks out of porn sites that are opened and tells me she doesn't know how they got there.

For the sake of this conversation, please refer to me as "The 8th Immoral".
That being said, If i had a "young friend" that watches porn and said "young friend" is a "she", i'd forsake g0d in hella seconds and "tear dat up".

Now on to your question/situation (The 8th Immortal persona taking over now)...
Generally, porn pop ups only occur if those sites are being visited.
Something just occurred to me. If your friend is on a site they may have "questionable content" and not necessarily porn, but maybe some type of adult rated info (the range varies) it is possible.
Let's say she's viewing a website, clicks on a link to another site or ad, something to that effect, then yes it is possible. Because i've had sites pop up behind what i'd currently be viewing and after i'd close the page i'm on, voila, there'd be that dreaded site featuring adult content.
Now, because i'm an advid porn viewer, a connoiseur of sorts, i notice a brief flash on the screen indicating a new window has opened. And, yep, you guessed it. Adult rated.

So, your friend may very well be viewing this stuff on purpose or not. Basically a judgment call is needed.