If I am being charitable I am going to slightly alter his argument by saying that bible explicitly forbids shrimps.

I don't know that this is being charitable. In this case to be charitable, I'd make the assumption that someone who told me God forbid them to eat something was drawing from OT law and not nitpick.

assuming that your interlocutor is a smart person capable of making good arguments without your help is a good principle to follow, especially as it is often true.

"Smart person" and "capable of making good arguments" are different things, and both are relative and open to many definitions.

As a former Fundamentalist Christian, I don't claim to be smart or very good at making arguments, but I'd say it is not a useful heuristic to enter into a debate or discussion assuming a sincere adherent of that belief system is capable of making a rational argument.