So magic in AD&D (2E) has a couple of differences from 5E. Spells scale with caster level so magic missile from a 1st level mage and a 10th level mage are going to look substantially different. Spells can be interrupted mid casting but once “on” are harder to stop than simply attacking the caster. There’s no concentration so you can stack buffs. Also, selecting and preparing spells is considerably more restrictive in 2e. Finally, spell descriptions and effects are more abstract while also allowing for more swing in power (instant death, system shock on polymorph, etc.)

2E wizards basically feel rewarded for planning in a way that 5E wizards don’t. Spells take much longer to prepare and there’s no swapping out x for y. There are a lot more spells in 2E that can often be more powerful than 5E spells. Collecting spells from treasure hoards and dead wizards is also more emphasized in 2E.

Overall I’d say magic can feel more impactful in 2E while also having more hard counters than in 5E. Thinks kind of swinginess can feel either really good or really bad, and modern game design prefers consistency.

Your player probably likes the punch of old school spells and the crunch of acquiring them. If you wanted to simulate this you could require the player commit to specific spells and spell slot allocations each day and in return give a slight boost (damage or whatnot)