[banned] got banned for helping a friend

Hello,
i got a question for you guys about a ban and if they can actually do something like this

prev week i gave 30k gold to a friend of mine where i've been playing with for more than 2 years so he could buy a gametime token ingame.
earlier this week i had a ban for this (through chat they said i got banned for giving away such an amount).

can they actually ban you for helping a player ? if they can then this would ban you too xD :
-selling items -> you help a playing obtaining an item
-raiding/dungs/boosting -> you help ppl in dungs to obtain gear
-pvp -> you help them obtaining pvp gear

with this in mind , would you need to play solo in an mmorph to receive no ban?

edit : just that you know , i never used a bot or exploit or anything else on this acc