@BetterInColour I don't quite get what everyone saying 'he's up to something, he'd never just want closure with an ex' want you to do.
Can't speak for the others but I wanted to suggest the OP be very wary of this man and his reasoning, something sounds off. It's a horrible confusing situation. Seeing friends is healthy, taking a break from endless rounds of weekends away is healthy, all that bit is fine, no issues there.
ExGF, by his own words, isn't a friend, and he's couching his reasoning to take EXGF out for the evening as, (and let's face it this is a bold brave stance), a benefit to the OP. It is not.
He is claiming it's only a psychological need, he isn't interested in ex at all, it's all about the OP, by using therapy speak to justify him taking his ex out for drinks. How often this will happen while DP and EXGF thrash out their individual needs for "closure" is anyone guess but this is the beginning of a new phase in OPs relationship with DP and again the OP has every right to tread carefully and not become so heavily involved or take a step back.
To an outsider this looks like manipulation and emotional abuse. "It's all for you love, I told you about it, don't complain eh?" Of course I could easily be wrong I don't know the man but I've met people who say these things, could be a coincidence.
If it is not manipulation, he could just be a thoughtless idiot, then this man has serious boundary issues regarding other women outside his relationships with any partner. Again I could easily be wrong, I don't know the man but I've met people who do this, again could be a coincidence.
You are right of course, you can't stop him doing anything he likes and there is nothing to OP can do other than to either accept it, be wary and watchful, or walk away. It's great she is catching up with a friend this weekend and taking a bit of a step back over the next few weeks. The whole thing sounds awful think the op protecting her emotions and taking a break from this man will be more healthy for her in the long run and absolutely the right things to do.
She sounds lovely and deserves to have a good DP whoever that is I wish her well and hope the best that can be will be for her