They could have had an argument then, or it could have been a reference to Jeremy taking her home after Colin's party. I don't think Sheila ever badmouthed Colin behind his back, so I'm wondering what the argument could have been about. They were divorced by that stage. It's possible Jeremy made a comment about Colin's alcohol consumption and Sheila retaliated by bringing up her brother's cannabis use.
I think maybe Jeremy looked down upon Shelia.
He had a car, a job, he was more confident than shelia. Shelia had mental health problems, couldn't keep a job etc et al.
Jeremy perhaps thought shelia was faking her mental health for attention