the present perfect?

Yes, it should be, “I met friends who I hadn’t seen in a long time.” This is Past perfect and the use of Simple Past “met” makes Past Perfect for ” hadn’t met’ necessary.

Some grammar text books try to explain the Past Perfect by calling it “Past Past!” and there is a good reason for this. When speaking or writing about two things which happened in the past, one before the other Simple Past is used for the most recent action and Past Perfect for the action which happenned before.

In your case The “not seeing” happened before you recently met.

An example of use of the Present Perfect would be, “I have some friends in London who I have not seen in a long time.”

I hope this helps.

Depends on the context. Are you talking about the present … ie, “Earlier today I met friends I haven’t seen in a long time.”

Or, are you telling a story about a while back, like “last year on my birthday I met friends I hadn’t seen in a long time.”

I think it’s an issue of how long ago you’re talking about. In the first sentence it would be a recent thing – maybe you used to see them regularly a long time ago, but recently you haven’t seen them much. And now you met with them again.

In the second sentence the entire thing is in the distant past. You hadn’t seen these people in a long time, and then you saw them…. no indication of where you stand with seeing them recently or in the present. It leaves open the possibility that now, you do see them on a regular basis. It’s just that back then, you didn’t see them much.

I think that’s the difference. Not 100% sure, been a while since I was in a grammar class… but that’s what sounds right to me. Hope this helps!

it works both ways,you should have some additional information about the present time and the time you are talking about.if you have met s.o. this morning (BUT IT’S STILL MORNING)you have to use present simple,IF IT’S NOW AFTERNOON OR THE NEXT DAY past perfect is more natural and even obligatory.

but you need time adverb like

TODAY I met friends i haven’t seen a long time(but it’s still that day)

or

at the party LAST NIGHT i met friends i HADN’T SEEN for a long time(till then)

you should ask your teacher for more clear explanation

i met friends i haven’t seen in a long time

It sounds more accurate, and frankly more normal, with the word “haven’t”. However, I could be wrong.