In order for the Israelis to strike at the Paki nuclear facilities they would need to have very very strong evidence that they were going to be giving weapons (not just tech transfers) to somebody like Hamas or Hizbollah or a "front line" Arab state. Absent an imminent direct threat like that, the negative consequences from even the most successful strike would outweigh all benefits.
Any Paki attack on Dimona would mean either a 1-way flight (they have no air to air refueling) or cooperation of Saudi Arabia or Jordan for refueling after the strike...which would make the refueling state a target for the Israelis. Also, Paki long range air strike abilities are way less than Israeli, and they would be flying in to an alerted and highly competent air defense system.