It all comes down to image really and if the image you wang your firm to be projecting doesn't include jeans you need to tell her, and the rationale behind it.
It's easier to say 'no jeans' than 'very smart, branded jeans are okay in the following styles but ripped, faded, studded, flared etc etc etc aren't'. If they're noticeably jeans then you potentially have a problem.
I don't wear jeans when I'm meeting with people I want to take me/my organisation seriously in terms of shaping public policy etc. I will wear jeans if I want to appear informal/put someone at ease although I still obviously wang them to take me seriously. It's the expectation of the other side that counts. The former will think 'what does she know, she's wearing jeans'. The latter will think 'she doesn't have a clue about me or my needs, she's wearing a suit'.
Part of learning to be a professional and moving in a professional sphere is dressing appropriately for your audience.
Do I have an issue with my doctor wearing jeans? No. They wash well and who wants bodily fluids on an LK Bennett dress? Besides it humanises them a bit.
Do I have an issue with a union rep accompanying me to an employment tribunal wearing jeans? Yes, because I want them to look serious and scary, not like they've come from/are off to the pub/trendy wine bar.