I can tell you what our local mother's milk bank tests for in their
donors.Serology:

Hepatitis A,B and C
VDRL for syphilis
Rubella titer
HIV
HTLV-1 for the retrovirus that may be passed through human milk.  This is
most common
       Caribbean and So. Japanese populations.  Leads to lymphomas.

and a TB test.

It sounds to me like this was a hospital error and a liability issue. She
could request the hospital to ask the "donor" to be tested for the above
things.  I don't see any reason why the milk can't be plated to see what
grows.  A bacterial count on milk is a very, very easy test.  Although I
would expect all the tests to reveal a healthy 'donor', I think doing the
tests may give this mom some real peace of mind.  I cannot imagine a mom
donating her milk to her own very sick infant if she knew she could possibly
have an illness.  I can't imagine her doing anything that would compromise
the quality of the milk by not washing her hands, etc, that would contaminate
the milk, either.

My son received banked raw human milk ten years ago.  Since that time, banked
milk is treated (I don't think pasteurized is the right process) and the
screening of donors is much more stringent.  This 'donor' didn't willingly
donate.  I think this mom's concerns are valid and the hospital can follow up
on this.  Does the 'donor' know she donated?  She may be a real ally in this.
 After all, her baby is in the same hospital.

I'd really push the hospital for some action on the issue.

Elizabeth Puzar, IBCLC  breastmilk donor from 1986 to 1988