I'm not sure if it's common to say "Generous to a fault". Would "Generoso a una falla" be the right translation? It sounds off to me. I'm looking for Hispanic use.
"He was generous to a fault" : A man so generous it worked against him.
The first thing tht comes to my mind would be
Demasiado generoso para su propio bien.
You could try to go with "demasiado generoso" alone, but I don't think that by itself ot has the connotations of "too much for their own good" (which would be "para su propio bien" in Spanish).
Similar with other possibilities like
Se pasó de generoso
Generoso en exceso / excesivamente generoso
It could just mean that he was too or very generous (going above and beyond) but not necessarily "too much for their own good".
generoso hasta lo nocivo / perjudicial / contraproducente
could not work that well, although they could be used. There is something tricky about "generoso" (or kind, helpful, etc.), since when you claim that it was "to a fault" you can't really tell if it was counterproductive to the person who was being helped or to the person giving help. You could go with most of these, but my preference is to go with the "demasiado X para su propio bien" expression.
Generoso a una falla is not idiomatic; however, I suggest interpreting this as pecar de generoso to imply to much generosity caused a problem.