If Jack is as much of an asshole to woman as it looks it's pointless to talk to him, so don't do it.
You don't know Paul so it's not your responsibility to talk to him. His relationship none of your business.
So the only option besides doing nothing is talking to Jan. Do it in person and do it privately. Explain to her how this whole situation looks like and tell her that she should stop (or take it easy if there's nothing going on) for the sake of her boyfriend and her reputation.
You can tell her what you've heard of Jack, but she probably won't listen, so all you can tell her is that it's her decision, but she really should make a decision rather than having her cake and eat it too. If she wants Jack, so be it, but she should officially leave Paul then.

Jan is the only person doing wrong here (if she is) so she's the one you should talk to.

Y'know, sometimes the most obvious thing is overlooked. Nuderaider is right, if you're gonna do anything, it should be to Jan.

Okay, I don't have any credentials that make this "my business", though you still haven't shown why it is "wrong" for me to share my concern, especially if I would want someone to do the same for me.

Dammit Cecil.

1. Who are you to judge another man's relationship when you're so new and naive to the scene yourself? How can you say a girl is being unfaithful to her relationship when you can't even prove your suspicion. How can you know that such actions aren't acceptable in her current situation? Maybe Paul and Jan have affairs with other people, but know about them, understand them, and accept them because they feel that such things don't get in the way of their relationship. What if Jan is just playing your "asshole" buddy for a fool because Paul overheard him saying some shit and thought it'd be funny to mess with him? You don't know what's going on. You are wrong to assume that you know how things should be. You are wrong to think that this is your business at all. You're not a counselor, you're not either of these people's friends or family, you're not anything.

2. Develop your own damn morals and learn to think for yourself. Asking internet people about such a dumb issue is pathetic.

3. What is "wrong" is subjective. I expected university man to know these things.... You disappoint me, cecil.

I meant morally wrong.

So do I. Your morals are not absolute or even relative to anyone outside your reality (unless they are allowing you to influence). You are in no position to say what is "wrong" in terms of someone else's relationship.

It's none of your business.

I sit and look out upon all the sorrows of the world, and upon all oppression and shame;
I hear secret convulsive sobs from young men, at anguish with themselves, remorseful after deeds done;
I see, in low life, the mother misused by her children, dying, neglected, gaunt, desperate;
I see the wife misused by her husband—I see the treacherous seducer of young women;
I mark the ranklings of jealousy and unrequited love, attempted to be hid—I see these sights on the earth;
I see the workings of battle, pestilence, tyranny—I see martyrs and prisoners;
I observe a famine at sea—I observe the sailors casting lots who shall be kill’d, to preserve the lives of the rest;
I observe the slights and degradations cast by arrogant persons upon laborers, the poor, and upon negroes, and the like;
All these—All the meanness and agony without end, I sitting, look out upon,
See, hear, and am silent.

I contacted Paul this morning

In my personal opinion, I should have just had a one on one talk with Jan before doing anything else, sharing my concerns with her.

Good job doing the wrong thing, too bad it didn't completely blow up in your face or you might have learned something.

You should've made this thread before doing anything, and then listened to Nude, who provided the solution of talking to Jan in the first place, and hence you could've taken that solution.

Perhaps I should be a detective as well.

Because you made a totally erroneous assumption about a stranger and then acted poorly based on it?

UPDATE:

Jack is pissed off and doesn't want to "ever see me again".

Looks like Jack had some plans that were ruined. Oh well, at least Jan and Paul are fine, as they were the ones I was concerned for.

Yeah Jack doesn't want to talk to me, but I want to talk with him. I don't like ending on a bad note.