My friend is playing a wizard/sorcerer (in a campaign I'm not in) and he's claiming this (irect quote from discord):
you can learn any spell by either finding a scroll or copying it from another wizard
you can be a level 2 wiz, 8 sorc and learn level 5 wizards spells
However, from what I'm reading...
You determine what spells you know and can prepare for each class individually, as if you were a single classed member of that class. (PHB 164)
and then also,
When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a level for which you have spell slots... (PHB 114)
It doesn't mention how the spellbook works with multiclassing specifically, but based on these rules wouldn't a 2 wiz / 8 sorc only be able to copy level one spells into their spellbook?