Dear NT:

You wrote:
> Which of the following are correct:
>
> 1-For him, drinking is reprehensible only when his enemies indulge so.
> 2-For him, drinking is reprehensible only when his enemies indulge in that
> way.
> 3-For him, drinking is reprehensible only when his enemies do so.
>
A lot would depend on context. All make sense, but #3 is the most natural sounding. With #2 there is a problem with the “that.” What specific way do you mean? It could make sense in context.

#1 is OK but perhaps a bit stilted. #1 & #3 would be better without the “so,” since there is nothing in the context to tell us what the “so” refers to.

I would drop the “so” in either #1 or #3 unless there was something in the context that gave us a clue what the manner of drinking (what the “so” or “that way”) was.