Allegedly, the mother who wrote the letter saw the story about the whiny Moroccan who killed her son on Flemish TV. That sort of extremely local, minor story is not at all likely to have made it into the English-speaking press. Therefore, it would not necessarily be so easy to confirm as you suggest; simple web searches using English vocabulary would probably not turn it up.
As for public records, they would contain multiple identities of convicted killers in Belgium's prison system, any number of whom may have participated in the prison violence (which the above link states did include prisoners setting fires). How do you reduce the possibilities to the right individual with any certainty?
And then there's the small matter of actually accessing those public records. If you know how to get at Belgian prison records, I'd be curious to know how it's done. Being an attorney, I might actually have a use for it someday.
As for the degrees of separation in the chain of communication, I have received assurances via email, though admittedly indirect and sanitized for safety, that the source is real but must remain anonymous. Obviously, that could be false information. Or it could be true. It's really a "jury" question. I can't speak for you, but my own direct contact with these people gives me a sense that they are telling the story the way a truthful person would tell it, and that above all they do not wish to endanger an innocent person. It's one of those things where people have to make up their own mind, but for my part, so far, I am inclined to believe it, though I do not begrudge you your doubt.
Assurances and “ifs” and “possibilities” doesn’t make it fact.
=8-)