You have received a lot of good advice so far. Sorry you are here, and sorry we are all here. I'll just emphasize my thoughts on some aspects of your initial posting.
my wife started an affair with a coworker of hers about 8months into our marriage
You have found out how she deals with life, and it isn't good, she deals with her "issues" by betraying those who trust her the most. RUN BEFORE SHE AND YOU HAVE KIDS TO DEAL WITH WHILE SHE BETRAYS YOU.
she had the affair because i wasn't sexually satisfying her
No she didn't. That is a cop out comment blaming you for her behavior. You could be a fucking Adonis and the greatest lover since Adam and she'd still have cheated on you. If she wasn't sexually satisfied, she could have worked with you on that and gotten satisfied. Cheating doesn't lead to satisfaction.
We decided to reconcile and we are on the right path now but it still hurts.
She's in full self protection mode. She is not protecting you, she is protecting herself.
the guy is married 8 years 4kids and he just had a baby earlier this year so he cheated on his wife with my wife their whole pregnacy it says something about them both.
Yes, it does, and think about that for a while. Do you want to be the spouse with kids and the cheating wife in a few years?
FWIW, I'm that guy 18 years ago, my wife cheated on me after we had 4 children, 18 years ago now, and put me through Hell when she did it and after discovery and D-Day. I couldn't figure out what was going on, I have 4 kids all under the age of 8, my wife is ignoring me, I can't figure out what is going on, my family is coming unraveled, and I'm trying to hold it all together and save my family. I'm wondering what I did that was so bad that my family is collapsing. That is what that is like. Let me tell you, his wife is in a special Hell, post-partum depression is the least of her worries.
he begged me not to tell his wife because shes suffering from postpartum depression
Tell her. I wish someone had told me so I would have been able to deal with the shitstorm of lies and sandbagging I was getting in a realistic fashion, it would have saved me YEARS of confusion and grief.
also me and my wife have no kids together. She is extremely remorseful and is really trying to help me heal.
She can't be that remorseful at this point, she can't have progressed that far in realizing her behavior's impact.
i really want to tell the guys wife
Tell her, and don't let your wife know before you do. Don't let him know before you do. Do it with kindness, honesty, and in the best way you can, but let her know what is really going on in her marriage. It will at the very least let her know that she isn't a failure in her marriage, it is her spouse that is the issue.
i feel really ashamed and embarassed what do you guys think what should i do?
Only you can decide. She used you and she engaged in the willful destruction of another family, what else has she done that you don't have a clue about?