Is a faux hawk hairstyle appropriate for a professional workplace?
A faux hawk hairstyle can easily be brushed back at work so that the employee would still have the option of gelling up after work.
On the other hand, I would not pick this fight with a good employee. Our hairstyles are a big part of our self-expression and identity. This is especially true for younger people.
So, unless there is a really good reason why the employee should not have the faux hawk I would focus on his positive attributes and leave his hair style to him.