Can my player’s multiclass Warlock 1/Sorcerer 8 cast the higher level sorcerer spells from with their warlock pact magic spell slots?
My player argued that he could now cast his 4th level sorcerer spells with his level 1 warlock spells slots. I think he interpreted pact magic as "highest spell slot possible", so he figures that since he has 4th level spell slots, he interprets that the warlock spell slots are 4th level slots now too.
A variant of this question has already been asked here: Paladin/Warlock sharing spell slots
The short version is you can't use a 1st level spell slot to cast a higher-level spell and as a 1st level warlock, the pact magic ability only gives you one slot of 1st level.
If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know. (PHB, p. 164)
You still have to follow the rules for spellcasting though.
All of your warlock spell slots are of the same level.
The Warlock table shows how many spell slots you have. The table also shows what the level of those slots is; all of your spell slots are the same level. (PHB, p. 107)
