"Which law is disputed ("broken" by Mr No-deal)?"
That will be the Benn Act, when the Queen has given her assent on Monday, September 9. Boris or any other PM will be in violation of that on October 20th if he has not 'sought and secured' either an extension or Parliamentary approval for a no-deal Brexit.

Boris Johnson 'will be forced from power if he defies no-deal law'

The conclusions of a team of leading QCs, which have been sent to the shadow Brexit secretary, Keir Starmer, make clear that the prime minister would be declared in contempt of court if he tried to remain in No 10 while refusing to obey legislation to prevent the UK leaving the EU without a deal on 31 October. The new law is expected to gain royal assent from the Queen early next week.
....
One of the QCs who provided the unequivocal advice, Philippe Sands, told the Observer: "If the prime minister chooses not to comply with EU (Withdrawal) No 6 Act, he will be subject to an action for contempt which could, logically and as a matter of last resort, lead to imprisonment, but that has never happened and will not happen; Britain is a rule of law country, so he will comply or leave office. All other talk is bluster, as attorney general Geoffrey Cox will already have advised him."

Geoffrey Cox is the Attorney General in the current Conservative Government.

In addition Parliament can select another PM, which might be necessary to gain the approval of the EU. A new PM would be a change of government, after all. But even so, there is no assurance EU agreement will follow.

"It is not necessary to have hope in order to persevere."