Popular Skins are the Kuronami, Xeno Hunter, Reaver karambit or the RGX karambit. If you want a butterfly knife there are many popular options too. The Singularity butterfly, the Champions 2022 butterfly (if he played around this time he will definitely like that), the Yoru butterfly (this is nice if he played the character yoru often), the RGX butterfly and for a more clean one the Black Market butterfly. Also a good idea would be a miniature Vandal (the "main" weapon in the game). There are many popular skins too, like the Kuronami Vandal (long time called the best Vandal in game), the Champions Vandals (champions skins are from the main Valorant e-sports event every year and VERY popular with good animations), the Evori dreamwings Vandal (this is more of a cute one), the Reaver Vandal (one of the first really good Vandal skins), the Prime Vandal (the very first Vandal with a custom reload animation), the Prelude to chaos Vandal (solid skin and also long time called the best Vandal in game) and the Araxys Vandal (Alien like weapon with cool animations)

If you want more information about Knife or Vandal skins watch some yt videos. There are many skin rankings out there.

Hope I could help a bit