Depends on why someone supported Trump in the first place.
For some it's how he pisses off the right people in their minds: They not only accept Trump's behavior, they embrace said behavior, for others it's his policy positions be it on an immigration, trade, economic or foreign policy level.
What's a red line depends on the person in question. Some, like myself, might be annoyed when he goes over the line with some of the things he says like attacking Comey or when he abandons his campaign promises to push a Republican establishment domestic agenda but that doesn't mean we'll stop supporting him, and maybe it's just me but since when is supporting a politician equal to you agree and endorse everything he or she does? What is Trumpism a religion now? Must I bow to the Altar of the GOP and Pray to Lord Trump for salvation to qualify as a supporter. Laughable.
My red line is on foreign policy: If Trump starts another moronic, narrow minded, stupid war driven by "human rights" or "spreading democracy" in the Middle East he can rightly fuck off, oh and that includes Iran by the way. Don't care how heated the rhetoric is though, it can be cool or scorching hot, as long as continues down the present course he'll be fine...if he changes though it will go badly.