What is the relation between Dr. Kureha and Gol D. Roger?
Maybe Roger had visited Kureha to cure his illness before he went to Crocus. Or Roger might have just saved her village/island back on his voyages. 

I am not so sure about their relation is, but they're definitely not enemies since Kureha smiled when stating D still lives,



Which telling us that she knows about Will of D and I am sure that in OP world, people who really know about it are always related to Roger.