I think she said she 'barely passed' in Potions. And it's not like he wanted her to help us actually brew, he said something about how trying to explain it to her might help us realise something we'd overlooked.

I could understand that if we were talking it over with Sirius, he's actually very good at Potions. Or even the twins, they're always concocting something for the shop. I don't know why it vexed me so much. Maybe--no, I don't--

You know what it was. He was treating her...differently.