Conflicts of Interest
Conflicts of interest are serious. If you have a conflict, you CANNOT collaborate with someone.
Not only is this bad form, but you hurt everyone involved. If you have a conflict of interest and violate that confidence, you could get in trouble (read: go to jail), you damage the person whose secrets you violate, and you contaminate the IP of the person you are helping.
It is bad all around.
The best etiquette is to just get up and walk away if you have a conflict of interest. Protect everyone involved, including you.