I receive ongoing Safeguarding training because of my work, and the mantra which is drummed into us is this:
Never investigate, always refer.
The OP doesn’t know the background, and doesn’t need to know - she knows what she saw and heard, which sounds awful. Every reason to be concerned. Not her place to decide whether intervention is needed or what form it should take. Absolutely right to report.
The policy these days is that any safeguarding concern, from whatever source, is logged at the Multi Agency Safeguarding Hub with as many details as available. Some incidents are so severe that action is taken straight away. Others are not, so they lie on file. And some weeks or months or years later, another report comes in about the same child. It all adds up, a picture emerges, and action is taken.
OP, you witnessed one piece of a jigsaw. You absolutely did the right thing in telling your DC’s teacher. If you want belt and braces, I would email the school’s Safeguarding lead and tell them you had spoken to the teacher, and add what you subsequently heard from your neighbour. As others have said, an email will ensure there is a paper trail, so no chance of it slipping the teacher’s mind. Plus you now have further information.