Backstory (I've a couple of other threads) - Ex left me when our DS was just 8mths old. Moved in with another woman obscenely quickly/there was crossover, and since then has either been a totally absent Father or a brief Disney Dad, dependant upon his mood...For context he's only spent, in total, no more than 40 hours with our child over the past year and a half.
Now two years on from when he abandoned us (after many months of infrequent visits, no co-parenting/financial support and total NC , for months at a time at his own volition) he's decided that he wants to be a 'Father' 
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The up shot of this, since January, (as I will never deny him access to our son - I hate him intensely, but love our child more), is that he now 'visits' our child for a few hours EOW. He collects him from my home (I can barely look at him), takes him out to soft play/park/creative play etc etc and then returns with him a few hours later...In between visits he gives me zero parental support, barely enquires after our child and pays (and only for the past 6 months) the minimum towards maintenance. I am a lone parent working a 50hr week and I'm currently a mixture of deep bitterness and sorrow for how life is for me right now (see other threads), combined with shear wonder and joy at the awesomeness of the child I gave birth to. But life is one long day of plate spinning.
My reason for asking for advice from you, is this - Last weekend the OW was (for want of a better word) stashed around the corner, unbeknown to me, and joined my Ex with our child on their outing. I only discovered this as our son (super bright two year old) dropped into conversation how she had been with them during the visit.
Basically (as he now lives an hour and a half from us), he'd driven over, dropped her around the corner, collected our son, driven back around the corner and picked her up and then done the same in reverse 3 hours later 
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I've never met her. I've never spoken to her. I think, though am not positive, this was the first time she has met our child. My ex is a total knob, a man child, and she is the woman who he has spent all his time with over the past year and a half and he has forsaken his relationship with our son in the process.
Please give me some advice as to how to address this clear underhand, cowardly and down right disrespectful (though clearly not unusual, given the cretin he is) behaviour they have BOTH shown me, so that when he arrives for his next visit I can be measured and dignified.
I know that I have zero say in who he introduces to our child, so please don't remind me of this (I am fucked off that she has, but they've been together for some time, so it was inevitable).
What burns is the manner in which it has been done . I just need some wise words from anyone please, given that in a fortnight I'll be staring at his smug face at my front door, wanting to pour bleach in his eyes, but also wanting to rise above the whole unnecessary 'drama' of it all. She of course may or may not be in the car when he arrives this time...
He spends such little time with our DS (just 18 hours so far this year), that to involve a third party at this stage just illustrates what a complete Bellend I'm having to continue to have contact with.
She's been introduced behind my back, I want to be dignified when I see him, but I also want to tell him that I know she's met our DS and that going forward she doesn't need to wait around the corner like a prostitute 
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