PC's casting spells on NPCs. What to do about it?

[BeefEater]

So I witnessed a guy cast invisibility on the farmer, Jorn who gives money for wolf pelts. I took a SS of my combat log with his name. I was invisible at the area transition when I witnessed the person cast the spell, so they probably had no idea anyone was watching. I tried to dispel the invisibility but was not able to. I asked what I should do in the DM channel but didn't get an answer so I just though I would ask here.

What should I do about it, for this instance and in future ones?

Thanks.

[Luna]

Send the screen shot to the DM Alias.
Then we can go deal with it.

Thanks