Advice needed!

So the other day I was going through my browser history looking for a webpage when I saw some entries for redtube. For those not in the know redtube is like youtube but for porn. Now I will not deny having gone there in past but not on the day that I was looking at in the history. After looking at the times and date I realized that on that day I had left my netbook at hone for my niece to type up a paper, because her laptop was on the fritz.

In Chrome opening a new tab displays your top nine most visited sites. While helping her mother look up something on my niece's laptop I noticed redtube was on the new tab page. I asked my niece to come help me with something in another room and told her what I had seen. Then I gave her tips on how to hide her viewing habits better and told her I wouldn't tell her mom.

Now I'm wondering if I made the right call. I'm thinking I should have a sit down conversation about safe sex with her to be sure she has good info. I am conflicted about not telling her mom. Should I tell her? I'm really unsure what her reaction would be.