wil leaving a rooster with hens for less than a day promise fertility?

The short and honest answer is NO. If the roo is shy or the hens want to get to know hi first, you could end up with what you started with. You would have better luck taking the hens to the Roo, but still at less than a day, there would be no guarantee. 10 days is the minimum I would leave them together for best results.

 

You are better off buying hatching eggs from someone else. Hens do not automatically submit to any old rooster; they often fight with him, injuring one or both of them.

Also, there is the disease factor. You could be introducing disease (you cannot always see a carrier situation like Mycoplasmosis or Coryza) to your hen or the hen to the rooster, even. It's not a good idea at all.

 

I have to agree with the others for the same reasons. You'll be much better off getting hatching eggs from your friend instead of borrowing his rooster.