Translation:They drank milk with their friends.
We cannot say "have drank" in English. It would be "They have drunk milk with their friends," which should also be an accepted translation, as is "They drank milk with their friends." They are both possible.
"Elles buvaient du lait avec leurs amis" is in the imparfait, and could mean "They were drinking milk" or "They used to drink milk" or "They drank milk." There is usually some overlap between the imparfait and the passé composé, which is why it is so hard for English-speakers to decide sometimes which one is appropriate.
Try reading up on the Tips and Notes for this unit, as well as for the unit of Imparfait.
"Drunk", not "drank", is the past participle of "to drink". Similarly, "gone", not "went" is the past participle of "to go". When I saw this question, I just knew some people would be complaining that the answer starts, "They have drank..." It drives me bonkers when people say things like, "I have went to Canada 3 times."
Yes. It is the past participle of boive (to drink). If you are using compound past tense, you always need a past participle to go with the auxiliary verb. (ont bu).
Perhaps you are thinking of the English sentence "They had milk with their friends." In French, you don't "have" food, you "take" it. Therefore, you would say "Elles ont pris du lait avec leurs amis." "Avoir" is about possession, not consumption.
I sometimes can't hear that the sentence is taking about "they" and not "her" when is it slowed down. The fast pace speaker is too fast and the slow speaker doesn't elude to the fact that it is elles and not elle. Elles--(s)ont bu du lait avec leurs--(s)amis. It still gets me every time.
If it was about "her" it would be "Elle a bu du lait avec ses amis." Completely different.
On the slow version, you lose the elision that causes the normally silent S at the end of of elles and leurs to be sounded as a "z."
The normal speed versions are not especially fast - keep at it and you will get used to it!