I'm certainly not a legal wizz but I am the HR Manager of a company that sounds very much like your other-halfs new employers, so thought I'd give my side of the argument. We also employ 'trainers and consultants' so think the idea is the same.
For us, a 'Trainer' is someone who travels around the country conducting courses at client sites, so the travel element is essential. I always make a point of mentioning at the interview stage the large amount of travelling involved in the role and ask if that would consitute a problem. Now, most people say "no, that's fine" and take the job. When it comes down to the week-in-week-out travelling however - they don't like it and start 'mentioning' how much travelling they are doing and the hours they are putting in for the company. I fully understand that for the majority of people being away from home loads is not a great thing - but I do point out that extensive was always part of the job spec and they knew what they were taking on. That said, was it specifically mentioned at the interview stage or highlighted as an issue for him to consider?
The way I combat it as a problem within our company is to operate a very 'informal' (i.e I can stop it at any time...) yet monitored flexi time scheme. That way, I recognise the commitment people like your DP are making but do not alienate the remaining office based staff who are also 'doing their jobs'.............
If he has only been there a short while and likes all the other aspects of the job/company - I would advise treading carefully. It may well be that rather than jumping in with "the working time directive says......" (which I have to admit would piss me off, if he had known the hours at the outset) he is better placed to ask for an informal meeting with his boss to say "I have a young family and as much as I love the job and working here, is there anyway we can look at all the travelling I am doing to make my work life balance better?". That way, he comes across as keen and flexible but also good at communicating and working through problems. You never know, they may well say - sure what did you have in mind............
Plus, any employer would be a fool to ignore a request like that, given the recent publicity given to life balance and the hours employees in the UK work...........hope that helps.