soft shell

Hi,  I have a problem with thin egg shells which break very easily when handled. This only applies to one hen, a pekin,  she hasn't been laying very long, her first egg was the size of a blackbird egg!  She doesn't lay every day, about once a week.  All the other 4 hens seems o be OK.   They have layers mash with oats and a few sunflower seeds mixed in every morning and then have their corn later nearer bed time. There is a pot of mixed grit and oyster shell available always.  I am slowly  switching over to pellets now the better weather is coming (we hope). They are free ranging for most of the day. I am worried the egg may break inside her, is this possible and should she have a supplement?

Any suggestions please.
Thanks.