Infidelity Support Group
Any relationship in which one partner engages willfully in sexual relations with another outside of the partnership is considered to have experienced infidelity. This breach of trust is often traumatizing for the faithful partner as well as the relationship, and support is often needed to heal emotionally and to decide whether or not the relationship should continue after...

You can be sure that since she's locked all access to her digital devices, she is actively partaking in her infidelity. As Happymama2j says, she is unrepentant, and this is the worst place for a cheating spouse to be, when the loyal spouse wants to reconcile, because it won't happen.
A cheating spouse needs to come clean, open up all their tablets, phones, online sites, etc. Nothing is private anymore, and shouldn't be in a partnership. She gave up her right to any sort of privacy when she decided to cheat on you, so her telling you crap about invading her privacy is horse manure. She's just mad that you found out and is trying to manipulate you by playing that card, and btw, a very common move amongst cheating spouses.
I suspect she's deeply involved in her indiscretions, having a torrid affair, tearing up love letters, taking day trips to see him and expecting you to believe she's visiting a friend. She'll never come clean in couples counselling, she's going just for you, in hopes that it'll be enough to keep you quiet.
You have some tough decisions to make, and like JustDrifting says, you either wallop her with consequences, or go for separation and divorce. If she cannot be truthful to you, become fully transparent, unlock and allow you to view her all her digital devices, you will never be able to trust her again.
I wish you well.
Unfortunately it is pretty common for your situation to happen. I went through a similar situation, and the solution was a different couples counselor. the original couples counselor was ok with her blaming me and then shutting down and not answering any of my issues or questions. I started talking to my individual counselor about it and he suggested that I try a different MC. the new MC did not allow her to get away with the shutting down.
I'm also dealing with a spouse who feels as though I'm invading his privacy when I ask him to be transparent. I've come to realize that means that his private life (whatever that may be) is more important to him than me and our family.
Regardless of what happened in your past, that does not give her the right to cheat. If she's done with your relationship, she should have asked for a divorce, not gone behind your back. She is not giving you or your relationship the respect it deserves.
You're not alone in this. No one wants to be the one to throw in the towel, but sometimes that's just what we need to do. Now I need to go take my own advice.
http://www.dailystrength.org/groups/healing-infidelity-through-faith/news/view/4008020
Provides 10 "steps" to follow in dealing with a CS who isn't cooperating with rebuilding in the hopes of moving them to being cooperative or at least preparing you if they remain uncooperative for the eventual divorce.
Hope that helps.
I suggest that unless she can be open, you stop the MC. it's a waste of time, because you have no idea what she's up to.
You need to prepare for a life without her, unless you are happy for her to continue like this.
If she suffers no consequences, why would she stop
If she knows you have no intention of leaving her, she will continue yo cheat and have zero respect for you,
It's time to take action, or let it be. What does your MC say about her locking phones and FB etc
How can you attempt to build trust back. It's thought to accept, but your reality is that she doesn't care about you , or she would not be doing this.
Cancel the next MC appointment
Consider IC for yourself
Look up the 180.
Stop showing any concern for her
If you have childten, sound time and enjoy them
Be prepared to loose the marriage or find another kind of happiness while you remain there.
You need to detach emotionally from her, do she can't hurt you anymore. She shouldn't be going to see thst friend thete out of respect for you , as it triggers you, but she doesn't care. She's probably still seeing him when she goes there. This affair is still active.
Get sad tested
Use condoms with her, she isn't a safe partner
Then decide how much of this mistreatment you are prepared to tolerate, because she's not sorry one bit and you are her doormat.
Stand up for yourself.
I would also add - seek a lawyer. Things will NEVER be the same again. Your wife threw your marriage away. How long will you be ok with that?