If you choose to confront her, I would just make sure your expectations are set appropriately. If you're expecting remorse, an apology, or an admission of guilt you won't get it.

Here's what will happen:

1) She'll deny it, the more you push the more she'll dig in
2) She will demand to know where you're getting your information
3) If you tell her you snooped, she'll get angry at you for that, tell you that you betrayed her trust and make you the bad guy
4) She will make an effort to lock you out of whatever you have access to.
5) She will deepen her relationship with OM in response to things with you getting worse.

That's what will happen, but that's not necessarily a bad thing -- you will need to make this worse before it can get better.

If you choose to confront her, here's what I want you to do:

1) Tell her you know about her relationship with OM and that you consider it to be a betrayal
2) You will not accept being with someone who is in a relationship with someone else, therefore you want her to (move out of the MBR, move out of the house, whatever you want)

(Think about what you're going to do if she says "no")

3) Think about anything else you want to do -- if she's texting him on a phone you're paying for, tell her you're going to cancel her mobile plan/phone and she can go get her own if she wants to use it to text with OM

The key things you want to go for here are to come across as showing strength and standing up for yourself. Secondly, you don't want to come across as controlling. The message to her is that she can do whatever she wants, but if she wants to keep up this relationship with OM then she will not be sleeping in a room with you, or using your mobile plan, etc. etc.

Establish some boundaries about what you will accept and then *stick to them and enforce them at all costs* it's the best thing you can do.

Prepare yourself for the fact that this will temporarily make things worse between you. Think about it and mentally prepare for it, how will you act after the confrontation? How will you handle yourself? Start practicing for that in advance. Strength is the key here.