Can you help a chicken absorb calcium ?

Recently we had a chook that was laying very thin shelled eggs and we were trying to make sure she ate plenty of calcium but then one day she laid just an egg with no shell, not a jelly egg, just egg no membrane or anything so we figured one of the thin shelled eggs had broken as she went to lay it and the egg came out leaving the rest inside her.  We waited a few days to see if she managed to get it out but to no avail.  You could tell she wasn't happy and was unwell and her poop was looking iffy so we took her to the vets who had a feel around and said that she could feel some lumps inside her and it wasn't looking good (had a few chickens with tumours so this wasn't a surprise) and gave us some antibiotics and anti inflamatory.  The vet said it may be that there was something wrong with the chook that was stopping her absorbing calcium hence the thin eggshells.  She is only 18 months old so a bit young to be getting ready to give up laying like the older ones do.

The good news is that after 2 days of medicine this morning she laid a rather solid and long lump of shell (probably more than one eggs worth) so she had managed to get rid of the blockage we think the anti inflamatory helped reduce any swelling inside her to help her get it out and seems to be back to her usual self at the moment.  But if she is having problems absorbing calcium we want to make sure that this egg breaking inside her doesn't happen again so is there anything we can give her that helps her absorb it.  I know with humans we have to mix our vitamins to help us absorb some more readily than others, and wondered if we could do the same for chooks. 

Thanks in advance

Sam

She has layers pellets and oyster shell available, and lots grass to nibble.  We are wary of giving her extra as we read that its not good for them, we just want to see if we can help her absorb it a bit better.  Most of our chooks start laying thin shells when they get near to the end of egg production, but with her only being about 18 months old I thought it was a bit early for her, but maybe she is going to finish early.  Just want to avoid getting the shell stuck inside her again as the lump she laid this morning must have been very painful for her.