That sounds like a reasonable use of Ready.
Here's the relevant rules text (my emphasis):
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.
A character can (usually, see below) perceive when another character finishes casting a spell, or when they signal that they have done so, such as by shouting "OK, go". Characters can use Ready to coordinate their actions provided they give each other that type of perceptible cue. Guidance specifically requires that the caster touch the (willing) target, so that is also clearly a perceivable circumstance.
How perceptible a spell is when being cast depends on its components, of course, so in oppositional scenarios, where a character wants to Ready with an enemy spellcaster's spell as the trigger, the DM will need to account for how well the character can see or hear the caster, whether the spell has verbal, somatic, or material components, whether modifications to those components are in use (via subtle spell metamagic or other effects), and so on.
In short, the trigger has to be something the Ready-ing character is aware of, but not necessarily a physical event.