Trust your gut!

I think its quite telling that he was trying to hide his phone from you. People with nothing to hide, hide nothing. I think its sad that he's flipping it all around on you and saying that his secretive behavior is just because you would be upset. Well, I think you have a right to be upset! Using the excuse of "why would he start now?" for cheating....?!?! That's as if to say that all cheaters are cheaters from day one. Nope! Some people are faithful for many years before they cheat.

I think that since you two are in a rough patch, he should be staying away from all other women, even if they are just friends. Why? Because he should be putting his efforts into YOU and you ALONE! Unfortunately, right now he's putting effort into this other woman (whatever she may be). So pretty much he's at work with this woman all day long and then texts her when he's spending time with you. Not cool.

As for this "woman friend" bit....is he the type of guy who has always had female friends? If the answer is "no"....then you know this woman isn't just a friend. Guys fall into one of 2 categories....those who have female friends and those who do not. When those who do not suddenly start making female friends....watch out, that's a red flag.