Google Fu is off because I can’t really think of the right wording (hence the problem).
The closest I’ve found so far is “When your opponent is drowning, throw the son of a bitch an anvil.” - James Carville.
But what I’m thinking of is a bit more benign… something like “when the other side is about to shoot themselves, don’t get in the way” or “If the opposition is about to self-destruct, don’t stop them”.
I’ve heard one that puts the above more artfully. Does anyone remember such a quote?