Hopefully you have the exact death date of Lewis and then you can request a copy of his death from the Fremont County Herald Chronicle, 44 N. Bridge, St. Anthony, ID 83445, 208-624-4455. His obituary might have her maiden name.
Check with the Fremont Court Clerk/Recorder, 151 West First North, St. Anthony 83445, 208-624-7401 and ask if th emother's maiden name appears on the child's birth certificate. It does in other states but I'm not sure about Idaho. Successful Searching!