I don't think a spell granting advantage on charisma-based skills is too overpowered, and encourages different ways of competing the game.
Most likely, a spell granting advantage on charisma-based skills would encourage a single way of completing the game - casting that spell for every single charisma check. Especially since it is trivial & consequence-less to long rest in BG3 and restore spell slots.
It'd also make all other spells/abilities that grant advantage on charisma checks (at a higher cost, level, or other penalty) less meaningful, as advantage doesn't stack.
What should be fixed is the discrepancy between the BG3 Friends tooltip and what actually happens. The tooltip says "Once the spell ends, the creature knows it was Charmed and might accuse the spellcaster." Keyword
accuse. Similar to what happens when you steal from someone, casting friends
should result in the NPC afterward coming up and initiating dialogue. The player has to persuade/bribe/intimidate the NPC in order for combat to not be initiated, and casting any spell during this conversation (guidance, Friends, etc) should result in automatic combat as the NPC is on the lookout for your magical mind control.
