He wasn't eve going for her at the time of the stabbing. I know you can also argue self defence in defence of a child but he wasn't even going to hit Henry or hurt him - he was "just" shouting and ranting.
So afaik, helen was (rightly), at the end of her tether but at the time it happened, she was leaving (so not weak, not a victim), she had cooked dinner for them both (ie was in her normal state of mind), there was a row, her son was there (so she didn't protect him from seeing the row), Rob was shouting at Henry but didnt go for either Henry or Helen physically, and she stabbed him twice or three times.
It doesn't look like a proportionate self defence response to me. Necessarily.