I apologise in advance if I repeat anything as I didnt read any of the replies
My warmblood can have the same issues. He's not a very forward horse at all. Usually to get him going I let him stand for a second and comb the reins through my hands so that he brings his head onto the bit (dont ask me how this helps it just has for me). Once his head is down he tends to start to listen a tad more. Sit there for a second and then turn his head to one side. While you do this let your leg (on the same side) put some pressure back behind the girth. Push push push push bringing his head around with the reins and pushing back behind the girth ALWAYS makes him move. As soon as he starts moving I release ALL pressure on him and let him take a step or two. Then repeat. Keep changing directions and push push push push push he needs to know that it doesnt matter what he wants, he will move. Don't do it for long each time though. Give it five minutes and if he responds well finish off for the day and do something fun with him. But don't get off while things arent going well. If things are sucky just keep going until you get that 10 seconds that are good. Then hop off. Always reinforce to him that he did well so he starts to learn that if he does what he is asked he will get relief from the pressure.
Having said all that there are other issues that you could look at that may be causing this kind of behaviour.
* poor fitting saddle - if the saddle doesnt fit right and is putting pressure on him anywhere he may be in pain and trying to tell you so by acting out
* teeth/bit - have the horses teeth been floated recently? Sharp teeth can cause pain for the horse although this is usually made known by head throwing but still something to think about. Also, what kind of bit do you use?
* feet/legs - are his feet well looked after and up to date with trimmings/shoeing? Does he have any injuries on his legs or past injuries?
* back/shoulders/hips/neck/poll - this can be another cause of horses acting out. Chiropractors work wonders
Does your friend ride in a relaxed 'soft' manner or is she tense because of these issues? Does she have soft hands?
Obviously if you rule out any physical or riders issues there is only one option left: attitude. If its attitude, show no mercy. I don't mean go flogging the living daylights out of him to make him go because this will sour him more. What I mean by that is you don't have to go easy on him thinking its a physical issue. But, this is ONLY if ALL physical issues have been addressed.
The main idea is probably to go back to basics. Start again with a bunch of groundwork and balancing exercises. He needs to learn to be supple and willing and groundwork is one of the best ways to do this. He also needs trust and a bond with his rider. He needs to want to please and to want to go. The 7 games by parelli would be a great place to start. It teaches respect, bonds and establishes the rider as the dominant horse or herd leader. This is the key. You need to become lead mare cause no one messes with the lead mare :)
There are some videos in the instructional videos section (video section) that outline some of the 7 games. I havent checked since yesterday but there were a few on there and im pretty sure appy would have added a couple more by now. They are a great tool in training horses right
Is there any chance of getting some pics or video of what she is doing?