How do you know it's non cancerous?
It should not hurt her unless too much pressure is put on it. I would also suggest that depending on what type of lump it is, she might want to start taking vitamin E. This helps with fybroids. Before starting she should consult with her doctor.
I have the same thing on my Breast and i went to a doctor about it and it turned out to be hard tissue and either way that never stopped me having sex, its not painfull, only if you stoke her breasts to hard lol.......
It shouldn't hurt her (unless you plan on putting direct pressure on it while you're having sex). I've never heard of sex having any bearing on the status of a breast lump. Are you sure it's not a cyst or swollen mammary gland?