Alfred Rowe -- a disgraced prison guard formerly employed at Camille Graham Correctional Center in South Carolina, where Smith served until 2000 -- tells TMZ ... the parole board made the right choice by keeping her in prison.

Rowe says he doesn't think Smith has changed ... and getting emotional at the hearing was all for show -- with AR saying someone must've bottled some tears and brought them in for her to use, 'cause she's not capable of that level of emotion on her own.

Rowe says it's possible she feels a little remorse ... but, her getting worked up like she did was a total fabrication.

The ex-guard previously admitted to us sex with Smith wasn't worth losing his job or his reputation -- he told TMZ he "allowed" Smith to give him oral sex 2 to 3 times, but said she asked for nothing in return. Prison authorities caught wind of their interactions and fired Rowe and another guard. Rowe pleaded guilty in 2001 to sex with an inmate and was sentenced to  5 years' probation. He said his negative opinion of Susan has nothing to do with their past.

As we told you ... Smith's appeal to the parole board was shot down unanimously this morning -- with five voting to deny and one recusing herself from the vote.

Smith claimed she'd learned a lot and changed since murdering her 3-year-old and 14-month-old sons back in 1994. The boys' father came to the hearing and advocated on their behalf --  saying this was a horrible decision, not an accident or mistake.

The parole board cited the seriousness of Smith's crime and her unfavorable discipline record as why they turned down her request. Rowe previously referred us to her disciplinary infractions over the years -- many for drug-related offenses. He noted her infraction record was among the worst he had ever seen.