You can follow with an actual Architect card, or choose to Think regardless of whether you have a legal action to play. I think I am understanding the rules correctly. Your opponent can lead with a Jack, because a Jack can be used to lead or follow any role.
Jack Fictional Roman citizens of all trades. Jacks may be used to lead as or follow any role.
The Leader selects a role, and then each other player gets the option of Following, or Thinking.
On each turn the player with the Leader Card (The Leader) decides what type of turn (Role) it is going to be, and then each other player decides whether to participate (Follow) or draw cards (Think) Here’s how to play each turn:
If the Leader “leads” then in clockwise order around the table each other player may either:
In clockwise order, each other player now gets to perform one action if they chose to follow, plus an additional action for each client he has of the chosen role. Players who thought instead of following do still get to take actions for their clients!
The decision to "follow" is made by playing the Order card, and doesn't require a mandatory use of the action. This is supported in the rules by the Stairway Card.
Stairway - Each time you execute an Architect action you may also chose to move an appropriate material into any opponent’s completed structure in order to immediately grant that structure’s function to all players ... You may perform the additional action even if you choose not to use your Architect action.