I know you asked about supplements, but here is my advice beyond supplements. 
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In the first few weeks: Make dinner, offer to drive her somewhere, come over and help out with any older kids, sweep her floors, do her dishes, etc. Bring over a great nursing pillow and help her with positioning. Things like that. She will be physically limited for a few weeks, and anything that can help in that capacity is awesome.
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Longer term: Listen to her talk about her birth with an open heart. Many mamas make an emotional journey during the year(s) after their section(s). She may embrace her section at first and then question it. She may never question it but still grieve over the whole thing. She may be 100% fine with it. Regardless, she'll want/need to talk about it to a person who is open to hearing her wherever she is on her journey.
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You're sweet for taking care of your friend. 