If I am a divination wizard, and I have someone take 5 for their saving throw, can they spend a luck point from their lucky feat to roll something else? It seems like they can I just want to make sure.
Portent overrides the Lucky feat
The Lucky feat reads (PHB, 167)
Whenever you make an attack roll, an ability check, or a saving throw, you can spend one luck point to roll an additional d20. You can choose to spend one of your luck points after you roll the die, but before the outcome is determined. You choose which of the d20s is used for the attack roll, ability check, or saving throw.
the Divination Wizard's Portent feature reads (PHB, 116)
You can replace any attack roll, saving throw, or ability check made by you or a creature that you can see with one of these foretelling rolls.You must choose to do so before the roll, and you can replace a roll in this way only once per turn.
spending a luck point expands the number of rolls you can choose from whereas Portent replaces the roll entirely. If a creature has advantage on a roll Portent does not just replace a single die, it replaces the roll entirely. Thus if Portent is used and a creature were allowed to expend a luck point on the roll the luck point would be wasted because the outcome of the roll has already been determined.
This answer covers the effect of Portent on Advantage/Disadvantage as well as with the Halfling's Lucky feature