"Matěj, you are a good friend."
Translation:Matěji, vy jste dobrý přítel.
Is it OK to use "přítel[kyně]" when talking about merely friends? I heard it means more like a boyfriend/girlfriend and simply friends are called "kamarád[ka]" or "kamoš".
It is ok, but you might get some strange looks. :) If you want to avoid that, use kamarád/kamarádka. Kámoš/kámoška is more like 'buddy.'
It is OK in this case. The word "přítel[kyně]" implies a closer and more trustful relationship than "kamarád[ka]". The sentence sounds very earnest, like something you would say after your friend would offer you help in a difficult time.
When speaking about boyfriends and girlfriends, possessive pronouns are usually present ("můj přítel", "moje přítelkyně").