Well, a couple of patchs ago they changed Zyra's plants interaction with Rylai. Previously, a plant auto was considered as an AoE for Rylai and such slowed by 40 % for 1 second. With the "bugfix", plant's auto are now considered as pet's attack, thus only slowing by 20 % for 1 second. Its efficiency has been halved on Zyra, however it still remains a good item on her imo, and I still build it every game because I love the Rylai-Liandry combo too much for my own good. :D (but it's totally ok to drop it alltogether, delay it in your build or just getting it situationaly, Zyra's build is pretty versatil after all)
On the other hand, Zyra synergise very well with Luden, and Liandry/Luden combo is fantastic for insane poking ability. It doesn't have the HP Rylai has, but the movespeed still helps Zyra dodging skillshots and escape more easily, which also helps her to survive.
In a nutshell your friend is right about Rylai not being the core item it was on Zyra anymore, and Luden is a good substitute, but it's still a decent choice. Moreover, the Liandry/Rylai/Luden combo is also very good, so you don't necesseraly need to choose between them. :3
(sry for the grammar and spelling mistakes, I'm sure I've made some. :c)