As a supernatural event that validates him and increases his insight, is it possible for a Wali (ally of Allaah The Almighty) to see the spirit of the Prophet, sallallaahu ‘alayhi wa sallam?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

The appearance of the spirit of the Prophet, sallallaahu ‘alayhi wa sallam, to a Wali and considering this as a supernatural event that validates him, is a groundless claim under Sharee‘ah (Islamic legislation). The foremost allies of Allaah The Almighty were Abu Bakr, ‘Umar, ‘Uthmaan, ‘Ali, the ten [Companions] who were given the glad tidings of Paradise, those who participated in the Battle of Badr and those who were present at the Pledge of Ar-Ridhwaan . None of them claimed that they saw the spirit of the Prophet, sallallaahu ‘alayhi wa sallam, or that they met him after his death, unless it was in a vision while they were asleep. Scholars agree that seeing the Prophet, sallallaahu ‘alayhi wa sallam, in a vision while asleep is possible and may happen to some people. It was reported that the Prophet, sallallaahu ‘alayhi wa sallam, mentioned that whoever sees him in a vision, has indeed seen him as if he was awake. In another narration, he added that Satan does not appear in his image. [Al-Bukhari and Muslim]

Regarding the belief of the ignorant that the Prophet, sallallaahu ‘alayhi wa sallam, attends their gatherings and they rise for him, there is no doubt that it is a manifest deviation. It is an even greater evil to claim that they see the Prophet, sallallaahu ‘alayhi wa sallam, while they are awake. This is a definite contradiction of the Book of Allaah, the Sunnah (tradition) of the Prophet, sallallaahu ‘alayhi wa sallam, and the consensus of the Ummah (Muslim nation). The dead will be resurrected from their graves on the Day of Judgment, not in this world; Allaah The Almighty Says (what means): {Then indeed, after that you are to die.  Then indeed you, on the Day of Judgment, will be resurrected.}[Quran 23: 15-16]

Furthermore, Ibn Hazm reported the consensus of the ’Ummah that the Prophet, sallallaahu ‘alayhi wa sallam, and all his Companions will not come back to this world until they are resurrected along with the rest of mankind on the Day of Judgment.

Contemplating the lives of the foremost of the allies of Allaah The Almighty — from among the Companions of the Prophet, sallallaahu ‘alayhi wa sallam -- makes it easy to realize the difference between supernatural events that support the allies of Allaah The Almighty on one hand, and sorcery and superstition on the other.

Had it been possible for anyone to meet the Prophet, sallallaahu ‘alayhi wa sallam, in order to receive guidance from him and increase his insight, as mentioned in the question, then his Companions would have been the worthiest of it, particularly during times of adversity and tribulation, such as during the period of disagreement regarding the Caliphate of ‘Uthmaan, and later ‘Ali .

It was reported that ‘Umar ibn Al-Khattaab mentioned that he wished the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, had not departed from them until he had issued a ruling on three specific issues. [Al-Bukhari and Muslim] If the Prophet, sallallaahu ‘alayhi wa sallam, had appeared to anyone after his death, he would certainly have appeared to ‘Umar Al-Faarooq . ‘Umar did not wish to receive rulings on these three issues in his sleep because he knew quite well that Sharee‘ah rulings are not to be based on dreams.

On the contrary, some Sufis claim that they received their daily Athkaar (remembrance) and initiation vows from the Prophet while they were asleep or awake. Seeing the Prophet, sallallaahu ‘alayhi wa sallam, while awake is impossible as we have already mentioned. Sharee‘ah rulings may not be based on visions, according to the consensus of scholars.

And Allaah Knows best.