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we need a quick response as we are playing now. If a squad of terminators with a librarian in targets my heavy destroyers. He shot a psychic attack with his librarian and destroyed the unit. Now he is saying because that unit is destroyed his terminators in the same squad are free to shoot at another unit

That is untrue, if it's a psychic shooting attack(and it sounds like it is) then it fires at the same time as all the other shooting, so you have declared a target for the squad already and all shooting from a squad happens simultaneously. See page 12 and 13 of the main rulebook, the relevant rule is on the top left side of page 13 in bold under the which models can fire heading.

If he had split fire he could have had 1 model target a different unit than the rest of the squad but would have needed to declare that before actually resolving any shooting or declaring targets. For split fire if they have the special rule see page 42 of the main rulebook.

If it was blood lance if i recall its not exactly a physic shooting attack as all the blood angel people will tell me.
it apparently chooses a point and roles 4d6 for distance. that's the only thing i can think of.

Blood Lance is a PSA, so it needs to target(for it it's target is the first unit the line touches) the same squad as the rest of the squads shooting. The first sentence of its rules actually specifically say "this power is a psychic shooting attack".