Bernie sells Protein Shakes and Nick sells the Chocolate ones after you reach 2 stars with him.
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Usually by giving them items that they want.
There is a link for a guide in the game description if you need extra help:
https://docs.google.com/spreadsheets/d/1nlO2owvibS5tvHbFyK83FGQc0PcGN16cWm7aq3Q4VLc/edit#gid=1919162492