My ruling is the condition can be as complex as the player wants, but they are still only allowed to ready a single action. So yeah they can build an "if/elseif" clause into their condition, but the action they take has to be the same regardless of which condition ends up getting triggered.
So they could say "if an enemy comes around that corner OR enters another party member's melee range, I will shoot my bow at them" (since it's the same action being taken regardless of which condition is met)
but they can't say "if an enemy comes around that corner I will throw my dagger at them, but if an enemy enters my melee range I will stab them with my rapier" (because attacking with a rapier is a different action than throwing a dagger)