It would probably be helpful to know how long ago she weaned her litter, but I have to say that for me it was quite a suprise how conspicous a cat's nipples were for some time after she had stopped lactacting. Sofie, who was 9 months old when I adopted her had very conspicous nipples for at least 18 months but this only became apparent when she was washing her belly. Eventually the hair seems to cover them over so they are less noticeable. I wonder if the owner has only noticed this 'tummy-washing' lately because her girl has been outside and got her tummy fur wet by wandering through grass? This may cause more frequent grooming which would of course reveal the nipples.
I would not be too perturbed unless it seemed that fur was being lost as a result of the grooming process or the nipples were inflamed in some way ... so I guess my advice would be observation and intervention if a problem seemed to be developing ...
And there's always Feliway if the owner is concerned about stress ...