that is really hard.... i don't really have any ideas except this one, and it may be laughable but i'll go ahead and say it anyway.... what about if he was the tv, like if u took a video of him singing her a favorite song or doing something really familiar, and played it at the same time each day like incorporated it into her daily routine.... like before bed or first thing in the morning or i dunno.... again its probably a ridiculous idea and i dunno at 6months if they respond to the tele, but my girl who is 10 months sure does so maybe its worth a try.... not sure if it would confuse her more though.
i guess when she is a bit older she will comprehend more that even though daddy goes away he always comes back.

my dd at the moment cries when her dad leaves for work for the day, so we go outside and wave him off as he drives off in the car, and it solves the problem... again, this doesn't really help you much, but i think it might get a bit easier as she gets older and understands more perhaps.

good luck.