I have no experience with this specific situation however I do have a suggestion that might help before you have the conversation.

I'd recommend drafting up a question and answer document. Try and anticipate the questions she will ask then write down the answers you will give. If you do this with your DH then that might help him deal with his fears about telling her. You might feel a bit dumb writing it down but it really does help. There would be nothing worse than her asking a question that you don't have an answer to. Or worse, you giving an answer that DH then contradicts you on.

Good luck.

Fiona