I think there are a few things at play here. The language you use in your OP is "that means not seeing DSS for nearly two months" which paints the picture that it is all about "his right" to see them.
Maybe you didn't mean that and maybe he doesn't see it like that. But added to that you seem quite casual about how he can't keep to his commitment because of work. It does sound a bit like double standards. The purpose of the contact order (or the agreement to joint residency if that what there is) is to protect the childrens right to time with both parents.
But I can't believe the comment about someone not understanding how someone with children could have a job that involves a lot of away time. On what planet does that make sense? A high percentage of business is international these days so if a person has a career (and presumably NRP is happy with the income that comes with that), chances are there will be travel involved.
I can't understand why in the world the RP, who has followed the order for 18 months, who just book a holiday without communicating with the NRP. It makes no sense and is highly inflammatory. She is in breach of the order, and no, he isn't in breach himself because as others said - it doesn't work both ways. With very good reason of course because you can be inflicting penalties on people for not seeing their children as you'd have unwilling and resentful parents in charge of children which wouldn't be fair.
Is there any reason you can see why she is suddenly doing this without discussion? I hope there's more to it. It infuriates me when people fail to communicate things they would naturally communicate with other people in their lives just because they "don't have to" or "why should they". It creates such a vile resentment between all parties that is completely unnecessary.
My advice would be for your husband to put in writing his proposal for making up the time, and see if she will agree to that. The children will enjoy the holiday and it seems very mean spirited to put barriers in the way of them going. Chances are, Mum will tell them that is what their Dad did and that contact time will not be very joyful at all! It's not right, but it seems the kindest thing for the children.
And of course document everything because if it happens three times then your husband may need to go back to court. Of course, if he regularly can't stick to his contact times then mum might ask for a review anyway.