I have to be honest, I don't know what, if anything, your H could say or do which will give you the comfort that you need that he will NEVER contact her. I don't think he could confidently tell you he will NEVER contact her again. It is like an addict saying there will never be a time when they want to take a hit again. What you have to ask yourself, is right, now are you willing to take it one day at a time.
He has to have the will power to say "No - today my actions will not be driven by my desires". And that is something only he can control. If he contacts her, you will never know. So you have to make a conscious decision to trust him (or not). If you do not trust him, then all his words and all his actions will make no difference. He sits on the toilet too long and in your mind he is texting, he goes for a walk, and in your mind he is texting, his phone beeps and in your mind it is her.
I am a smoker. Each time I quit, I count the days, then the weeks, then the months since my last cigarette. Then one day something will happen which makes me pick up a cigarette (it's mainly H related, but sometimes it's as simple as being out with friends), and then I start counting days again. Even after months and months of not smoking, I will always say I am a smoker who has not had a cigarette today. The desire will always be there. But, I can choose not to act on them. In the first month I see smokers everywhere. In the second month, I might only see them on those times I would normally have a cigarette, in the third month, I barely notice at all unless something reminds me (e.g. someone offers me a cigarette at the pub or my H and I have had a particularly difficult interaction).
