Yes, I think I would want to be told about that. Especially since there are health risks re. faeces, eg. hepatitis.
As people will no doubt say, tread softly and try to establish whether the T.A. corroborates that she was informed. If it's affirmed, then establish if the teacher was informed, and so on.
If it's not confirmed that the T. A. was made aware ... you have a tricky situation. See how they deal with it. I'd expect the teacher to gently tell your ds, right then and there, that things like this should be told to an adult.
Obviously, you will then have the issue that your ds says he did inform T. A. and T. A. saying he didn't to deal with ... .
In that situation, I'd tell my dc to tell a teacher and not the T. A. in the future, just because it cuts out any future difficulties.
If T. A. and teacher were informed and no "accident" report made its way back to me, I'd be looking at what guidelines there are about health issues like this and whether the school has contravened any.
Fwiw I can't imagine a school where this wouldn't be taken seriously.