In a large comprehensive a Governor really would not know anything about an individual child except perhaps for prize-giving, sport achievements and those written about in a news item. In other words, the same as everyone else.
It is likely that even if the Mum became a SEND governor, they would not know about individuals unless the SEND was very obvious, as it can be. But then again, it is obvious to everyone. A governor who knew a student that was excluded should not be on that Exclusion Committee and in order to avoid that, an exclusion committee is drawn from a larger pool of Governors.
It is worth knowing that Governors deal with strategic issues and not individual children. They cannot go into a school and look up children's records but they may be shown their books as part of a learning walk but often a school would avoid showing the governor a book of a child they know - there are plenty of others to choose from. All questionnaires you return should be unnamed and although the data is collected, it is non-attributable. Governors never discuss individual children and are concerned with whole school performance and improvement, not that of an individual child.
Where there is greater difficulty is in very small schools where there is a tight village community. It is much harder for governors, especially parent governors, to be unaware of the children in the cohorts, eg the SEND children, the PP children etc but professionalism and anonymity should be maintained at all times in discussions and minutes.
I am sure your friend will be professional and that the GB are too.