Coins can be bought in the shop quite cheap, but you do not.
Diamonds, however, are hard to get in the game and should be saved. Every now and then you need this mandatory. For example, to his castle upgrade in order to have more monsters.
With the friendship code you can add friends to his list, then when hopefully the torches to light (from Level 10), just as you should then do this with friends. Burning torches increase the chance to breed a rare monster. But with 10 torches are required min. 10 active friends!
5 diamonds may be obtained when a Friendship Code enters as Referral Code Options and these dispatches. The friend will then receive also 5 diamonds, and therefore it is highly sought after on the Internet that you will be entered there.
If you want to add me to his Freundschalftsliste ... or even a referral ;-) can use this code: 14351533CH
Have fun while playing.