Alexander Maconochie Centre prisoners Trinity Cook, 34, and Jaiden Dale Gardner entered the victim's prison cell in March 2020 when Gardner struck the victim a number of times to his head and face.

Gardner, 26, then poured boiling water over the victim's upper body resulting in serious burns that required transferring him to an interstate burns facility, an ACT Supreme Court judgment published on Monday states.

"The offender [Cook] watched on. He had known an assault was going to take place," it states.

Cook, who the court said has a criminal history in the ACT and a "significant criminal record" in NSW, pleaded guilty to being knowingly concerned in another person causing grievous bodily harm.

Justice Michael Elkaim sentenced Cook to a suspended 18-month jail term on the condition the offender enters a good behaviour order for the same period.

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The judge said he considered the offending to be about medium in the range of objective seriousness.

He based his assessment on the nature of the offending, in particular the pouring of boiling water causing serious and lasting injury, and the pre-arranging of the offence.

"To his credit, the offender did intervene to bring the assault to an end and he gave the victim a towel before departing," Justice Elkaim said.

"Had the offender's role not been passive, my assessment would have been higher."

The judge said deterring others "plays a significant part in offences committed in custody".

"Persons considering carrying out assaults in prison must know that they are going to be dealt with severely," he said.

Cook, who was born in Sydney, is required to follow any ACT Corrective Services-directed orders to undertake mental health counselling as part of his sentence.

Co-offender Gardner pleaded guilty in November to intentionally causing grievous bodily harm and is scheduled for sentencing in March.

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