I would suggest contacting the following parties for options:
Her oncologist.

A university cancer center or department of Pharmacology
They may be willing to collect the milk and do the testing to record the
levels in the milk.  This information while very interesting will not be good
unless it can get published somewhere so it will be available for others to
look up.

The Company that makes the drugs.  They maight be interested in measuring the
effects and the levels in the milk.

Try Dr. Tom Hale on LACTNET if he has any other suggestions on approaches to
this problem.

If she starts chemo before arrangements are made, I would still save the milk
and freeze it. Freezing might affect some of the drug level monitoring tests
but is probably better than not freezing.  The following information would
need to be noted. TIme drug is given, dose of drug given. Time/date milk
sample is collected, volume collected.  Finding what the levels do over time
after drug administration would be helpful information.

MY humble suggestions,

Jon Ahrendsen MD FAAFP