If you haven't touched it in 3 months, have no plans to use it in the near future and really want a Vita, then go for it.
Why not keep them both? Between the two consoles you literally can have a hundreds of games, and don't even get me started on how newer and better things are being done for the consoles as well (as far as hacking and homebrew goes).
I am assuming he doesn't have the money for a vita and needs to sell the 3ds to be able to afford one. So having both wouldn't be an option.
only problem would be that the charger port broke, so i have that nyko adapter thing that connects to a custom battery.
