Refraining from Salah due to denying its obligation is considered Kufr (disbelief), by Ijma‘ (consensus of scholars), and refraining due to negligence and laziness is Kufr, according to the sounder of the two opinions maintained by the scholars, because the Prophet (peace be upon him) said: (Part No. 18; Page No. 286)
“Between a man and Shirk (associating others with Allah in His Divinity or worship) and Kufr is abandoning Salah.”
(Related by Imam
Muslim in his "Sahih [Book of Authentic Hadith]")
He (peace be upon him) also said,
“The covenant that is between us and them (the hypocrites) is Salah. Anyone who abandons it has committed Kufr.”
Imam Ahmad and Ahl-ul-Sunan [Abu Dawud, Al-Tirmidhy, Al-Nasa’y, and Ibn Majah] with a sound Sanad [chain of narrators])
There are other textual proofs that support this also; it is therefore not permissible
to give a Muslim woman in marriage to someone who does not perform Salah. Allah (Exalted be He) says (what means),
O you who believe! When believing women come to you as emigrants, examine them; Allâh knows best as to their Faith, then if you ascertain that they are true believers, send them not back to the disbelievers. They are not lawful (wives) for the disbelievers nor are the disbelievers lawful (husbands) for them.
You have to advise them and clarify the ruling to them. May Allah guide you, them, and us to the right path! May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.