Arkadaşıma hoşça kal dedim.

It is:

• arkadaş = friend
• arkadaşım = my friend
• arkadaşıma = to my friend

So the word contains both the possessive ending -ım (my) and the dative ending -a (to).

Because the person receiving what is said is marked with the dative case in Turkish.

A very common pattern is:

birine bir şey demek = to say something to someone

So:

• arkadaşıma dedim = I said to my friend

Yes. Here it clearly means to my friend.

The -ım part is the 1st person singular possessive ending, so it shows that the friend is my friend.

Compare:

• arkadaşıma = to my friend
• arkadaşına = to his/her friend or to your friend (depending on context and form)

It is a fixed farewell expression. Literally, it is something like stay well or remain pleasantly, but in real usage it functions as goodbye.

The important cultural nuance is that hoşça kal is usually said by the person who is leaving, to the person who is staying.

Kal is the singular informal imperative of kalmak (to stay / remain).

So hoşça kal is literally an instruction or wish to one person: stay well.

If you are speaking:

• to one person informally: hoşça kal
• to one person formally, or to several people: hoşça kalın

Dedim comes from demek / de- (to say).

It breaks down as:

• de- = say
• -di = past tense
• -m = I

So dedim means I said.

Because Turkish often leaves out subject pronouns when the verb already shows the person.

In dedim, the -m already tells you the subject is I. So ben is unnecessary unless you want extra emphasis.

• Ben dedim = I said it / I was the one who said it
• Dedim = simply I said

Turkish usually puts the verb near the end, so this order is very natural.

A literal structure is:

To my friend + goodbye + I said

The direct speech expression hoşça kal comes before dedim, and the recipient arkadaşıma comes before that. English word order is different, but Turkish often builds the sentence this way.

You do not have to use quotation marks in an everyday sentence like this, and you do not need diye here.

All of these are possible, but the plain version is very normal:

• Arkadaşıma hoşça kal dedim.
• Arkadaşıma, “Hoşça kal,” dedim.

With demek, direct speech can simply come right before the verb. So the sentence is perfectly natural as written.

They are both farewell expressions, but they are used from opposite sides of the situation.

• The person who is leaving says hoşça kal to the person staying.
• The person who is staying says güle güle to the person leaving.

So they are not fully interchangeable.

You would usually change the farewell expression to hoşça kalın.

Examples:

• Arkadaşıma hoşça kalın dedim. = I said hoşça kalın to my friend.
  (formal or polite singular)
• Arkadaşlarıma hoşça kalın dedim. = I said hoşça kalın to my friends.

So the farewell changes depending on whether you are addressing one informal person, one formal person, or several people.