It might make more sense to use the "ty" form, considering the speaker and Matěj are přátelé.
"ty" form is accepted as well but to a native czech speaker this does not sound weird at all. The formal/informal addressing has many layers and it is common to call somebody by their first name yet use the "vy" form. Typical for colleagues at work for example.
The speaker merely states a fact. Maybe they are just colleagues with Matěj and the latter showed that he is somebody else's good friend prior to this conclusion.
OK, so normally, friends wouldn't address each other formally, in Czech, even though the vocative case is used? Am I understanding this properly? So if Matěj is the speaker's friend, the sentence would be, "Matěji, ty jsi dobrý přitel," correct?
BTW, this is so similar to Ukrainian!
Matvíju, vy/ty [je] dobryj pryjatel'.
Матвію, ви/ти [є] добрий приятель.
When you're addressing him directly, you use the vocative case. It's strange for English speakers because it doesn't exist in English.
Přítel means often "boyfriend" if a lady or woman says that about a man.
Kamarád means now just a friend, used to be (and maybe it still to some extent is) more colloquial than přítel, and used to be used especially in certain contexts say 100 years ago, but is now often used instead of přítel when a confusion with the "boyfriend" meaning is to be avoided.
I am having a lot of trouble trying to pronounce přítel. When ř is in Kateřina, it sounds like a r followed by zh. But when I listen to přítel, it sounds like pleetel, only not quite. I'm not hearing the zh at all.
There is no zh, it is a special single sound. Check the introductory Tips and notes and there are also some tutorials for pronunciation on Youtube.
There are two Ř sounds, one is voiced and one is unvoiced. Kateřina has the former one, přítel has the latter.
To my ears, it can sound either as "rzh" voiced or "shr" unvoiced. That's how my brain processes it.
Thank you both VladaFu and va-diim. That does help, to know that the consonant sounds different because it is unvoiced. I looked on Youtube and I was surprised and pleased to see that there are several videos entirely dedicated to pronouncing that sound! I can at least roll my rs, so I will practise.
I realize there was not a question mark, but don't you say it the same way whether a question or a statement?