What is the common way to say that someone's fast becomes invalid?
You could say someone "violated" their fast.
I know only "to observe a fast" and"to break a fast".
What I mean is when someone does something against the act of fasting, his/her fasting will become invalid. In other words, s/he cannot continue with his/her fasting practice until the last day of the fasting month. I'm not a muslim, but it's good to know about others' religious practices and able to speak about them in English.