I have experience on both sides of that matter of a six-year-old being separated from her mother, but neither involved anything that was essentially the fault of the mother. So, I suppose whether my suggestions are any good or not depend on whether the mother managed to still be a good influence on her child or not.
Anyway, my mother was hospitalized for seven/eight months just around my seventh birthday. My sister and I waited for her to be able to call and talk, and that meant so much to us. Also, we got to go stand outside the TB "san" and visit with my mother, who sat in her window. Staying in touch with her was absolutely the most important thing to my sister and me. My brother was a toddler, so that was a whole other thing. Keeping our minds occupied by going to school, playing with friends, having good times with our father, were all extremely important; because at night, once it was time to try to sleep, my sister and I pretty much cried every night as we worried about our mother.
Basically, we acted OK during the day, but that unhappiness was always there somewhere in the back of our mind, and at night when we had time to think and worry it would move to the front of our mind.
That experience helped me have a pretty good idea about how my own two youngest children felt when I took my three kids and left my marriage one day, and a couple of relatives thought I was "nuts" for doing so. Someone called in the state, and from there a whole bunch of lies were entered in court - and in spite of a clean bill-of-mental-health being presented in court. Their father moved with them to another part of the state, and it was longer than eight months. What was bad was that I had been the parent with whom they were closest and most comfortable, simply because that's how it often is when fathers work long hours and leave the "child matters" to the mother.
I'd say - again - that finding ways to keep in touch and still remain her mother, if she's at all able to be a strong, reassuring, positive-attitude mother who also knows how to get her child's mind off any gloomy thoughts, is the important thing.
I don't think the mother's absence should come without warning.