Can a guy wear a woman's helmet safely?
"Women's" helmet is a pure marketing feature. All helmets have to pass the same safety standards and if it fits wear it. 

Most helmet models come in at least 2 or 3 sizes. You should be able to find a helmet in the size that fits your head diameter. All the major manufacturers make helmets for every head size from 2 year olds to XL for adults. Every helmet should
have the head size range listed somewhere on the box.