his application for a renewal was refused
On what grounds? Has he completed his studies? If a decision has been made for his visa to not be renewed, I am presuming there is a good reason as to why this is. If you want people to help, you need to supply more information. Alternatively, search the forum as this type of question is often asked and you might find some answers in other threads.
him being currently an illegal visitor, would any employer hire him and help him get a visa?
I would think it highly unlikely. As you pointed out yourself, he is now staying illegally in Poland. Hiring him is just going to be hassle for an employer ( and possibly illegal considering his status ), and why would they bother when they can hire someone Polish who doesn't need a visa?