How can he advise someone who is falling short with regard to prayer?
You can remind this person and advise him by explaining the ruling on not praying, and on not praying in congregation in the mosque, and explain how the early generations of Muslims were with regard to prayer.
The ruling on not praying:
In a number of questions we have explained that the one who does not pray is a kaafir who is beyond the pale of Islam; no good deeds will be accepted from him on the Day of Resurrection; and his marriage contract with his wife must be annulled if she does pray. This one who does not pray should also understand that any meat slaughtered by him cannot be eaten; and (if he dies) he cannot be washed, have the funeral prayer offered for him, or be buried in the Muslim graveyard.
It was narrated from Samurah ibn Jundub (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) mentioned a dream that he had seen and said: “Last night two people came to me and said, ‘Let’s go,’ so I went with them. We came to a man who was lying down, with another man standing over him, holding a big rock. He threw the rock at the man’s head, smashing it. The rock rolled away, and the one who had thrown it followed it, and picked it up. By the time he came back to the man, his head had been restored to its former state. Then he (the one who had thrown the rock) did the same as he had done before. I said to (my two companions), ‘Subhaan Allaah! What is this?’ They said to me: Move on…”
At the end of the hadeeth, he (blessings and peace of Allah be upon him) said: “They said to me: ‘The first man whom you saw with his head being smashed with the rock, is the man who learned the Qur’aan then abandoned it, and he went to sleep, neglecting the obligatory prayers.’”
Narrated by al-Bukhaari, 6640
Muhammad ibn Nasr al-Marwazi said:
I heard Ishaaq – i.e., ibn Raahawayh – say: It was narrated in a saheeh report from the Prophet (blessings and peace of Allah be upon him) that the one who does not pray is a kaafir. It was the opinion of the scholars from the time of the Prophet (blessings and peace of Allah be upon him) that the one who deliberately does not pray, without any excuse, until the time for the prayer ends, is a kaafir. The time for the prayer ending means delaying Zuhr until sunset and Maghrib until sunrise.
Ta‘zeem Qadr as-Salaah, 2/929
With regard to praying in congregation in the mosque:
Just as the prayer is what differentiates between the Muslim and the kaafir, praying in congregation is the sign that differentiates between the believer and the hypocrite. The Sahaabah (may Allah be pleased with them) regarded the one who did not attend prayer in congregation as a hypocrite whose hypocrisy was known.
Shaykh Saalih al-Fawzaan (may Allah preserve him) was asked:
I am a young man, not married, and I pray but not continually, i.e., I do not always offer the prayer on time, and sometimes I miss the prayers for a whole day, or I do them all together. What is the Islamic ruling on that?
The Muslim is required to offer the prayers regularly on time, with the congregation of Muslims. It is not permissible to delay the prayer until its time is over. Allah, may He be exalted, says (interpretation of the meaning): “Verily, the prayer is enjoined on the believers at fixed hours” [an-Nisa’ 4:103], i.e., it is enjoined at specific times, at which it should be offered. According to a report: “Allah has deeds at night that He does not accept during the day, and deeds during the day that He does not accept at night.” Delaying the prayer until the time for it is over is neglecting it. Allah, may He be exalted, says (interpretation of the meaning): “Then, there has succeeded them a posterity who have given up As-Salat (the prayers) (i.e. made their Salat (prayers) to be lost, either by not offering them or by not offering them perfectly or by not offering them in their proper fixed times, etc.) and have followed lusts. So they will be thrown in Hell, except those who repent…” [Maryam 19:59, 60].
Putting two prayers together is not permissible, unless there is a legitimate shar‘i excuse at the time of one of them, such as putting Zuhr together with ‘Asr, or Maghrib together with ‘Isha’. But putting the prayers of an entire day together is not permissible and prayers are not valid if done in this manner.
Al-Muntaqa min Fataawa al-Fawzaan, 5/56, 57
With regard to how the early generations of Muslims were with regard to praying in congregation in the mosque:
They regarded it as something of the utmost importance, to which they paid a great deal of attention and were keen to offer the prayer in congregation in the mosque even if they had excuses not to do so.
Wakee‘ ibn al-Jarraah said, narrating from al-A‘mash Sulaymaan ibn Mahraan: al-A‘mash was nearly seventy years old but he never missed the first takbeer.
Muhammad ibn al-Mubaarak as-Soori said: If Sa‘eed ibn ‘Abd al-‘Azeez al-Tanookhi missed the prayer in congregation, he would weep.
It was narrated from Muhammad ibn Khafeef that he had a pain in his side that, when it affected him, prevented him from moving. But when the call to prayer was given, he would be carried on a man’s back. It was said to him: Why don’t you take it easy on yourself? He said: If you hear (the words of the adhaan), Hayya ‘ala as-salaah (come to prayer) and you do not see me in the row (of worshippers), then look for me in the graveyard.
‘Aamir ibn ‘Abdillah ibn az-Zubayr heard the mu’adhdhin when he was dying and he said: Hold my hand (and take me to the prayer). It was said: You are sick. He said: I can hear the caller of Allah; should I not respond to him? So they took him by the hand and he joined the imam in praying Maghrib; he prayed one rak‘ah then he died.
For more information see the answer to question no. 47123 – very important.
And Allah knows best.