How can I support a loved one who has had a miscarriage?

Be there for her. Miscarriage , can be very stressful.See what she need and be there for her by word and action.
Listen. Offer sympathy and understanding. Avoid cliches, platitudes, or judgments. Sometimes saying nothing, just being there and listening is best. Let them know it's ok to grieve. Each of us will go through the process differently and take a different amount of time. Frequently everyone forgets dad, but he's hurting and trying to be strong for his partner.