Resolved questions
When can you say, 'this person is a true friend'?

Is this question a grammatical question? Or is it a social question?

If it is a question regarding English, the meaning is roughly this:

"This person has proven that they are trustworthy and loyal over a sufficient period of time."

If it is a social question you ask, I suppose the answer is somewhat the same. Personally, I feel that someone is a true friend when he/she is trustworthy and loyal, EVEN IF THIS HAS ILL CONSEQUENCES to the person. For instance, let's imagine that a friend of mine meets an important business person named Mr. Jones for a job interview. Mr. Jones owns the company where my friend wants to work. During the interview, Mr. Jones sees my name on the resumé and says, "Oh, you know Erik Tyler? Well, that certainly isn't going to help you here. I think he's an egotistical, self-centered person." A true friend would speak up and politely disagree (e.g., "I have known Erik for 10 years and haven't found that to be true about him."), even if it meant losing the job opportunity.

That is only my subjective measure, but perhaps will be good consideration for others.