I'm not sure if that's really "breaking the game", or if the game was already broken with bloat from rules that WotC added over the years...
I'm not sure if it was broken from the bloat or whatever - class "balance" is really only an obsessive concern in games like MMORPGs. Tabletop games have always had much more relaxed "balance" concerns - D&D 3.5e was never truly meant to be a head-on challenge, and while 3e and 3.5e made more of an effort to reach a kind of balance, the concern was more than each class could contribute equally to an encounter, rather than be balanced in 1 vs 1 fights.
As for Druids the class balance for them and Clerics have always been pretty extreme in 3e and 3.5e mostly because players hated playing those classes (I still do) so they buffed them to insane levels to try an entice people to play them and the two classes are easily the most "overpowered" classes as far as individual power goes. I still hate playing Clerics and Druids though - I can't stand playing a character that is so totally beholden to a deity.
By 10th level though, especially fully-buffed, I'd think a wizard would actually last longer than just a single round, though.
Similarly, if spellcasters are allowed to chose any "legal" spell to have their books, warriors and similar equipment dependent classes should have an unlimited budget for magic items, since that's where a lot of their power comes, in particular, miscellaneous magical items. To limit equipment-based classes while giving spellcasters unlimited spell selection is very badly slanting the contest in favor of spellcasters already. And what would you do about Paladins and their warhorses? Or a Wizard with a summon or gate spell? Could such characters already have their summonings there? If not, you're biasing the game in favor of the druid again.
I can't imagine if groundrules like were followed that your Druid would have gotten away so scot-free without any "serious challengers."