As answered by Joshua Aslan Smith, it's not an automatic hit/ Saving Throw Fail.
The Spectator stat block in the Starter Set is actually missing a crucial part which can be found in the DM's Basic rules or the Spectator's entry in the Monster Manual:
... If the spell forced a saving throw, the chosen creature makes its own save. If the spell was an attack, the attack roll is rerolled against the chosen creature. (Basic Rules(DM), p.44; Monster Manual p. 30)
The unique ability is silent on whose Attack Bonus, Save DC and Spellcasting Ability is used. However, there is guidance in the Ring of Spell Turning (DMG 193), which has a similar effect:
... if you roll a 20 for the save and the spell is 7th-level or lower, the spell has no effect on you and instead targets the caster, using the slot level, spell save DC, attack bonus, and spellcasting ability of the caster.
The ability would suggest that it follows similar rules to the Ring of Spell Turning, and it makes sense that way since it is the same spell being cast by the original caster. The idea is also in-line with other effects that "hold" a spell's energy like the Ring of Spell Storing (DMG 192), where a spell retains the bonuses and DC of the original caster.
As for rerolling the Attack Roll, the Starter Set stat block is missing it again:
... If the spell was an attack, the attack roll is rerolled against the chosen creature.