Kashamu said Sheriff, who walked out of a meeting presided by GEJ, did not disrespect Jonathan but was understandably angry that he was not recognised as the party leader.
Kashamu faulted Jonathan for presiding over a meeting of the party where he said Ali Modu Sherrif was "disregarded" as the National Chairman of the party despite his authentication by a competent court.
Speaking in an interview on Sunday, Kashamu said the incident would be a setback to the ongoing reconciliation efforts in the party.
The Senator said Sheriff, who walked out of the meeting on Thursday, April 6, did not disrespect Jonathan by the action but was understandably angry that he was not recognised as the party leader.
Kashamu said: "I am honestly shocked and surprised that the former president could preside over a meeting where the judgment of the court of appeal was being disregarded as irrelevant.
"This cannot be the face of a party that is desirous of seeking a break with the past. Going by the accounts of what I have heard happened at the meeting, I think it was a needless gathering that has reopened healing wounds and further exacerbated the crisis. How could you gather over 400 people for a reconciliation meeting and expect any meaningful deliberations?
"I say this because any PDP meeting that fails to recognise what our laws and the courts have said about the leadership crisis is on a wrong footing and premise, and it will breed more chaos and confusion such as we have seen.
"Senator Ali Modu Sheriff is the national chairman of the PDP as of today based on the court of appeal judgment and everybody is bound to abide by that whether we like it or not. You cannot call a shareholders or board of directors meeting and say the chairman of the board cannot participate or preside over the meeting.
"Would the governors have allowed it if they got to the meeting and they were not so recognised? Or were they not addressed as governors even when their elections were being challenged in court? Worse still, some persons said Senator Sheriff must not be recognised as such even when an appellate court has established that Senator Sheriff is the national chairman of the party."
He also slammed the Ekiti State Governor, Ayodele Fayose, saying he is an agent of "destabilisation" in the party.
He said: "I had always known that Fayose has an agenda and alerted our leaders and elders to his agenda of either having his way or destroying the PDP before he leaves for God knows where.
"Now that he is letting the cat out of the bag, I can only say that Fayose has exposed himself.
"He has shown that he is the real agent of destabilisation. He is one of those who want to kill the PDP before he defects. "However, it should be clearly pointed out that PDP was not founded because of Fayose or any of us. Whether anyone leaves or not, the party will remain. It will not die.
"The truth of the matter is that the leadership crisis in the PDP was designed and orchestrated because of the inordinate ambition of some people, especially those who are exiting office next year."
The lawmaker added that with the road map for peace that Governor Seriake Dickson's committee has outlined, the onus lies on the elders in the party to execute it all sense of forthrightness and sincerity of purpose.