It means a male friend. That may be said in a way that cleary you think he is that kind of "friend" and in Polish przyjaciel/przyjaciółka is more close than English friend. But it is a word that describes friendship, not romantic relationship. So duo is right to not accept it.
Przyjaciel is a close friend that happens to be male. It is a friendship not a romace
Przyjaciółka is a close friend that happens to be female. It is a friendship not a romace
Chłopak is a boyfriend ( a romantic relationship) Dziewczyna is girlfriend ( a romantic relationship)
But chłopak/dziewczyna also mean older girl/boy, so that depends on context.
Generally yes, but some verbs in English are so-called "stative verbs" and they rarely are used in the Present Continuous, and if they are, they often change the meaning. In English, "I see Marta" and "I am seeing Marta" mean different things, regardless of the Polish sentence. "I am seeing Marta" most probably means that I'm dating her.
Not in this case:
To see / to be seeing is an unusual English verb (a stative verb, says Jellei) whose Simple Present and Present Continuous have very different meanings:
• to see: to see (by eye); to understand
• to be seeing: to be going out with (UK); to be dating (US).
So an native English speaker (in the UK, at least) might be surprised to hear you say I am seeing Marta with her friend, which implies you're an intimate threesome. Possible, of course—but you'd maybe only want to say so if it's actually true (and if all three of you are ready to come out…).
Disappointingly, the Polish widzieć just means to see (by eye).