First of all, how doez her wife's "conduct" negate the fact Garcia was never given his right to due process?
Due process does not mean a 10 month long jury trial, it means 'the process that is due'. A full jury trial and conviction is not part of the 'due process' afforded or appropriate to the person and the circumstances, in this case an illegal alien in immigration court determining his deportation status. He had his day in court with an attorney on this matter, he was able to appeal the matter, and it was upheld. That is the end of it, there is no more judicial review available.
As an illegal alien, he was entitled to a hearing with representation before an immigration judge. At this hearing, it was found that he was a member of MS-13, this is known as a
judicial finding of fact. This was a finding by the immigration judge supported by sufficient evidence, was able to be challenged by Garica/his attorney, was appealed to the relevant court, and that determination was upheld.
The docket is listed here, you'll find these documents attached as part of the initial finding.

The process that is due to an illegal alien was given to him, and appropriate to the context.
You have to remember also, 'due process' also does not mean 'he gets to have as many court cases as he wants in front of whatever judge he wants until he gets a favorable outcome', which is what the media keeps crowing about.