What he's doing is absolutely not right. Being 14 a little more recently (I'm 19), I can say that at that age, you do feel like you're an "adult" even though you really have a long way to go. I remember knowing a few people that did have flings with people in their early 20s. They were very flattered by this new attention and were probably easily convinced to do whatever they did with those people.

I can see that "Meg" is a little more level than that because you said that there was evidence that she shot him down a few times... but I wouldn't say that she is immune to being manipulated by "Erik".

I think that you need to confront Erik alone first. Tell him that you've seen the conversations and that you're very concerned. Be harsh, tell him that what he's doing is illegal and can put him in jail. First, make sure to give him some very valid threats.. like kicking him out, telling Meg's parents or even the police.. Make sure he deletes her number and then give him a chance to cease contact. Maybe even watch while he sends her a text along the lines of "we shouldn't talk anymore". Then give him a chance to change. You may even want to include your roommates in this confrontation.. if he knows that everyone is on to him.. he might give it up.

Of course this could just mean that he'll try harder to hide it from you. Just be very vigilant and do what you can to gain access to some of his conversations... maybe even check in with Meg a few times. If it seems like he's still doing it.. if you have proof.. you need to follow through on your threats immediately. Show him that you will not be lenient about an issue like this and that you are very serious about what you say.

I'm sorry that this is happening!