You can grow and harvest, and re-veg, and grow to harvest, over and over. I have done this for approximately 2 years. I believe “Spider mites” did that plant in. in the end.
You can leave a marijuana plant in the perpetualveg photo period of light, 18/6, and plant will not flower. This is how many growers keep “Mom’s” for years. Taking cuttings to clone a plant over and over. Ever hear of “Clone only” strains?
Your friend is closer to right than you are. Yes; You plant veg, then flower. and most of the time is stripped down for harvest. This kills the plant!
In order to keep a plant alive after harvest; You must carefully remove buds and NOT “leaves”. Leave the foliage. If you take all the Kolas off and trim back plant to at least 2 nodes with leaves per stem left on plant, You can re-vegetate that plant. You need the foliage to allow the roots to put out new growth.
Outside. I believe it is possible to allow a plant to die, over winter, and it will regenerate in the spring. My Habanero peppers do; As do most cultivars that fruit in the greenhouse.