Yes Shaz that was the shittiest thing my DH did to me, took the kids round to OW's when I didn't know there was an OW.
She will not be at all secure in their relationship, she knows that you have to have loads of contact because of your DS, she will become more controlling of your H no doubt, asking him all sorts of questions about what you and him talked about etc. Trying to put stipulations on his contact with you etc (this is what my DH's OW did).
The best thing that happen with me was when I found out what DH was up to (he had been gone for 6 weeks) and who she was, she sent me some texts along the lines of "you are so pathetic, your H no longer loves you or wants you, get over yourself babe (yes she put that in) and move on". Well, not knowing my DH was with an OW, I had had sex with him a couple of times over that period and we had exchanged emails leading up to the events, you get my drift. I texted her that my H had wanted me the previous Friday and if she cared to sent me her email address I would prove it , she did and I sent the emails that were obviously "undoctored" - just proved that my H was lying to her as well. Obviously by this time, I didn't particularly want my DH, already been to solicitor etc. But I was so pleased to have burst her bubble - she never texted me again after she received those emails .
Your H is probably trying to live with the guilt now, hopefully it will eat away at both of them, they deserve nothing less.