I agree with Pat that it may be a matter of wait and see.  My background is as a cardiac ICU nurse and IBCLC.  

One influencing factor may be the approach of the surgeon - they can approach through the sternum or between ribs depending on the preference and surgery.  If the intercostal method was used, then I would be concerned about the nerve damage which may impact on lactation.  Look for scars around that area - they probably won't be too difficult to find.  There may have been cardiac drains that are used around that axilla area which may be evidenced with scarring also.  You might also like to ask about how things went post-operatively.  Although it is unlikely, if there was extensive infection and she needed a muscle flap done, there may be damage to tissues and nerves.

I have recently worked with a mum who was breastfeeding and underwent cardiac surgery.  Besides needing to maintain supply by pumping/dumping for a while, lactation went on uneventfully after the surgery.

I'm sure there are plenty of other influencing factors and look forward to other replies.

Best wishes for your care of this mum and baby.

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