Multiple pimple different sizes 1-3 cm together on the infra scapula reg. of patient has metastatic breast cancer , could be related to cancer ?

Possibly: Breast cancer can metastasize to skin - see your oncologist.

Possibly: It is difficult to say without more information. These could be a reaction or a side effect of treatment, but breast cancer can recur as skin lesions that can resemble small pimples. These should be examined by her oncologist. A simple biopsy may be recommended.

Possibly: Hash the bast and safest thing to do is seek out a board certified dermatologist. These may be simple pimples, a sign of immune changes or others. If possible and they look like a cancer then a simple biopsy would give you all the answers.