mwana wa nzako ndi wako yemwe ukachenjera manja umadya naye

A child of your friend is your child; if you take care of him or her, you will profit.

What it means
The proverb treats a friend's child as one's own. Caring for that child is expected, and the saying adds that the care given comes back to the one who gives it.
Chewa · Southern Africa
Source and context
From: Chewa, Southern Africa
Source: Chewa/Chichewa (Malawi) oral-tradition proverb (mwana wa nzako ndi wako yemwe ukachenjera manja umadya naye). Earlier recorded here as documented in zambianobserver.com's Chewa proverbs page; that page, fetched directly (all 30 entries checked), does not contain it. The proverb is independently attested elsewhere with the same wording.

The meaning given here is our own reading. Other interpretations exist.