Assalaamu `alaykkum.
If they die as someone who is seen as a "Muslim" in the sight of Allaah, then they go to jannah inshaa'Allaah and this is something that is known. As mentioned before, we cannot make judgements on specific individuals.
With regards to missing Salaah, then it should be known that the person who leaves salaah is a "Kaafir" and there is ijmaa` (consensus) on this matter from the Sahaabah. And it is the highest level of Ijmaa` too!
The ijmaa` is based on the report narrated by Imaam At-Tirmidhi and Al-Haakim on the authority of `Abdullaah ibn Shaqeeq: "The companions of the Prophet :saw2: did not think that neglecting any deed made a person a kaafir, apart from neglecting the prayer.". Both At-Tirmidhi and Al-Haakim classed this narration as Saheeh according to the conditions of Al-Bukhaari and Muslim.
Ibn Hazm rahimahullaah said that it was reported from ‘Umar, ‘Abd al-Rahmaan ibn ‘Awf, Mu’aadh ibn Jabal, Abu Hurayrah and others among the Sahaabah. He said: “We do not know of any opposing view among the Sahaabah.”
Al-Mundhiri narrated this from him in Al-Targheeb wa’l-Tarheeb, and added more names of Sahaabah: ‘Abdullaah ibn Mas’ood, ‘Abdullaah ibn ‘Abbaas, Jaabir ibn ‘Abdullaah and Abu'l Dardaa’ [RadiaAllaahu `anhum].
Obviously the absence of disagreement (from other Sahaabah) shows agreement.
And there are various other narrations, like for example the narrations the sister above posted on the authority of Jaabir and Buraidah radiAllaahu `anhumaa.
We also have the narration of ‘Awf ibn Maalik that the Messenger of Allaah :saw2: said: “The best of your leaders are those whom you love and who love you, who pray for you and you for them. The worst of your leaders are those whom you hate and they hate you, whom you curse and they curse you.” He was asked, “O Messenger of Allaah, should we not fight them by the sword?” He said, “No, not so long as they establish the prayer among you. If you see anything on the part of your leaders that you dislike, denounce that action but do not break your allegiance to them.” [Muslim, 3453] and this hadeeth becomes more clear when the narration of ‘Ubaadah ibn al-Saamit is applied [Bukhaari and Muslim], when the Messenger of Allaah :saw2: told the companions not to go against the ruler and then said : "....unless they made a blatant show of kufr and you have evidence from Allaah that what they are doing is indeed kufr."
And the evidences are various. But what can be said, as mentioned by the people of knowledge, that 'one should not be going around making takfeer on people who do not pray since we must be giving the general people excuse of ignorance etc'
Wassalaamu `alaykkum