What Causes Trembling Hands In A Person Post Spinal Surgery?

Yes, pls. My friend had a spinal surgery about 12 days ago (scraped the vertebrate to stop compression which was causing numbness in one leg and foot flopping when walking. Anyway... he just got the stitched out (about a foot) 2 days ago. Just a little bit ago... he was texting me and his hands starting trembling.. and he could barely type. He said besides that, he felt fine. That lasted a minute or two... then he was able to type perfectly fine. ????? Ideas ???