There are certain ways from which your friend can buy you rank first of all is from the store of Pika Network, he can purchase gift card and give you the code of the gift card which you can use to buy ranks from the shop, or if he/she has a rank voucher in game, they can send it to you. Although the rank voucher thing cannot be used for PracticePvP or BedWars as they are not a thing there. Another way can be your friend can login in the store using you Minecraft username and directly make purchases to your account.