I have a dog with Separation Anxiety. It has taken from May this year to now to be able to leave him for up to two hours without him starting to get distressed and howl. (He started doing it when we lost our other dog to cancer in the Spring.)
The rehabilitation programme for SA is hard work and very intensive. It involves re-conditioning the dog's response to both you leaving and your not being there - if that makes sense - and the most important part of it is that you can never leave the dog alone beyond their tolerance level throughout the course of the programme. I religiously record my dogs when I'm out to make sure I'm not pushing him past his limit, as if I ever do, it will set his rehabilitation back considerably.
I have been literally housebound for months on end whilst rehabilitating my boy, the only time I went out was either to walk him or if dh was home and he could come in the car with us (for some reason he doesn't display SA in the car).
An awful lot of people simply wouldn't be able to fit the intensity of the programme into their lives and others may have the opportunity, but would resent the very real and significant restrictions it places on their lives. I'm lucky that I am a SAHM and we live across the road from my dcs' school, because at first he would howl within seconds of me closing the door. It has taken a lot of hard work to gradually build up his tolerance level, sometimes by only a minute at a time over a matter of weeks and we've had plenty of set-backs and regressions along the way.
Numpty that I am, I have now taken on a rescue pup who also has a degree of SA, so having dealt with our elder boy we are now going through something similar all over again. 
I hate the thought of my dogs being distressed when left and also of my neighbours being subjected to their howling - to the extent that I won't even leave them for a school run (usually between 10 and 15 minutes) without making sure they are settled with a stuffed treat toy that will last while I'm out.
Six hours is far too long to leave a dog alone without someone coming in and at the very least letting him out and spending some time with him - preferably taking him for a walk. Even if he isn't distressed when she first leaves, as ButThereAgain said, his stress levels will be elevated and rising the whole time she is gone.
I can highly recommend the book Don't Leave Me! by Nicole Wilde if you would like to suggest it to her. It helped us work out that our old boy had something a little different to the usual SA, in that he actually has Isolation Distress - rather than true SA, because he's not over-attached to us and his happy as long as he's not completely on his own. As a result he is pretty much cured now we have another dog ... it's just a shame the new pup doesn't see things the same way! 
If it's just her and him, it might help if she were to employ a dog walker to take him out once or twice a day, as not only is making sure the dog is properly exercised an important part of the programme, but having someone else in his life might help him to reduce his over-reliance on her a little - it would definitely be worth a try anyway. Even better would be dog daycare if there is one locally. It can work out expensive, but she would know that he is well taken care of and not spending the day pining for her and he would be building up some independence - again to reduce his over-reliance on her.