A friend is in your DB's situation. That friend is a woman - she is not primary carer. It's sheer hell for her, and I do feel for you, but the bottom line is that she has realised she just has to accept bad behaviour from her ex with grace, because every bit of anger or brinksmanship ups the ante and harms her daughter. So she doesn't ask for contact outside agreed occasions, and avoids telling her ex when something special is planned so it can't be sabotaged, and tries to comply with every stupid demand
The best advice, sadly, is not to seek legal help unless contact is actually blocked. If he is getting every other weekend then the best thing he can do is ask her to go to mediation with a view to making the relationship more amicable for the sake of the children, and to getting a consent order rubberstamped after the negotiations to make bloody sure he keeps his weekends, or she's in contempt of court. And if they talk in a more reasonable way, face to face (he has GOT to keep his temper) she may agree to the midweeks being reinstated. You don't know. If she doesn't, he doesn't have to go for the consent order but it is worth another try. (Why didn't they get one last time they mediated, as part of the divorce agreement?)
This is normal, a lot of the time. People with kids usually stay together until things become really unbearable and after the split they start to demonise one another in one another's minds.
Maybe you could email her and just say how fond you always were of her, how much you miss her (mention a few positive, non-linked-to-drama events) and how sad you are that things are getting so polarised. That she is always someone you'll see as family and if there is ever any way you can help she can always get in touch.
Basically fair or not, his contact with his kids will largely be down to goodwill unless you litigate. Litigation will guarantee they hate one another permanently, and will scar the kids badly - possibly, even probably, for life. If there is any way he can bite his tongue, not ever argue, just suck whatever up and placate, appease and accept, then there is a chance things will calm down and she will be more reasonable. That is screamingly unfair, but the thing is, she is the resident parent. Anything you ever so that upsets, angers or challenges her will affect the kids badly. That is not fair, but it's the case. The conflict and her anger will probably mean they get upset after contact, which she will interpret as contact being what has upset them, so she will want to limit it. If she relaxes because she is feeling less threatened and encroached upon, she will probably be less hostile in future - apart from anything else, it means more freedom for her, too. If she carries on incrementally limiting contact then litigation is the only way but I can't stress enough how toxic that will be to the kids. If he is getting every other weekend that is horrendous for a parent, I agree, but I very much doubt he's likely to get enough more from a court to justify the trauma (and cost. It will be thousands, and public funding is really, really limited now). High conflict residence cases are judgement of Solomon territory and there are no guarantees at all that he'll get any more time. If he is reliably getting every other weekend and she doesn't cancel regularly for no reason then she is not really seen as obdurate. The courts see implacably hostile mothers who barely allow contact at all, or who refuse point blank. And some solicitors are aggressive because it earns them more money, bluntly. I can recommend one who specialises in high conflict contact and residence disputes who dials down the drama at all times - a former CAFCASS officer I know recommended her to me for my friend when the need became inescapable, and she has been phenomenally good. She's top rated in Chambers and Partners for a reason. This is a molehill that could so easily be converted into a mountain - if he wants a good relationship with stable, happy kids, then whether they see their GD on a single occasion is something he MUST let go. It is not worth the long term cost.
Taking kids out of school without the other party's sayso is aggressive. Sorry, but it is. He needs to back off or he is going to wind her up more. The brutal fact is that this is not about who is right, or being fair, or "wins". It is about what is best for the kids. He needs to turn the other cheek, rise above it, suck it up and cope or he WILL see less of them. I know that's horrible but it's the truth. The other truth is that this is a game of tennis and each escalates it every time. He needs to disengage, at least for 6 months or so.
I think it's totally fair to want kids back by 5 on Sunday, by the way. They need to unwind and relax and prepare for the week ahead at school, and they need to settle back into their home. The travelling is a PITA and I sympathise, but it's not their doing and the fact is, that is a reasonable expectation. (They are collected from school on the Friday though, right?)
Again, this isn't about fairness or what's right or justice. It's about the best interests of two kids who have parents who appear on a collision course. Frankly, if more parents stopped worrying about their rights or their pride or what was fair and accepted that their nightmare ex just needs to be pandered to for the next however many years, then fewer kids would suffer. Because I do suspect that both sides see the other as monstrously unreasonable, and they have probably convinced their own sides of the same.
I'm really sorry. This must be awful for you, as well as the four of them.