* "They acted stupidly" is a bit blunt and judgmental. It implies that the actions themselves were inherently stupid, rather than just being unwise or poorly thought out.
* "They acted in a stupid manner" or "They acted foolishly" are more neutral and less harsh alternatives.
If you're aiming for a more formal tone, you could consider:
* "They acted unwisely."
* "Their actions were ill-advised."
* "Their decision-making was flawed."
Ultimately, the best choice depends on the specific context and the tone you want to convey.