If you know the name of the medal, someone at the French military archives may be able to find your ancestor on a list. The trouble with French military records is that they aren't on microfilms. Only trained researchers are allowed to browse the actual files and transcribe (no photocopies) the information from the record. So you need to hire a reputable researcher in France. I believe the archives can hook you up with a reputable researcher. Don't know what the cost is, as I haven't gone this route yet myself, but I hope perhaps this winter.