IS this person bipolar?

When we try to diagnose people with little to no information, the results can be in the very least awkward and embarrassing, but can often also be very very harmful.

What struck me first is that you seem to have a serious lack of information. You are not intimately familiar with bipolar, it appears, so trying to label someone with that is not the best idea.
Even if you know for a fact they are diagnosed bipolar it doesn't necessarily mean they are manic, or even having a depressive episode in the clinical sense.

Yes, we have mood disorders but not all of the moods we experience are disordered.

Why wouldn't you talk to someone if you care about them and they seem sad? Why do you have to focus on the sex aspect at all? If someone seems sad I feel like that should be the biggest concern, not that they may or may not be looking at racy pictures online (and how is it you know that, btw?).

Sadness is a basic human emotion that we can all relate to on some level.

To respond to the question of "is this a manic episode?", I'm not sure anyone can tell you for certain. But manic episodes with accompanying hypersexuality don't involve symptoms of depression or sadness. Even mixed episodes... the negative feelings expressed are not often described as "sadness" in my experience.

And "hypersexuality" seems to have become one of those catch-all terms, now evidently meaning anything from being a bit more aroused than usual to the specific temptation for (and giving in to) infidelity to describing blatant promiscuity to staying up all night watching porn because you were bored and had nothing to do all the next day.

No. Just... No.

Please just talk to your friend.
If you care about them, ask them if they are alright- that's pretty easy to start with.