Maybe my Japnese level isn’t high enough, though I don’t have this problem in other content so I doubt it, but the use of pronouns in the later mini stories is confusing or outright wrong. In the questions section it seems to be constantly jumping between multiple people’s perspectives, making it extemely difficult to follow. I am on the second part of story 53 and I am hit with ’ B)ケンドラと彼らの友達は15年間レスリングのチームに所属しています。’ I have no clue what 彼ら is doing here, who does it mean? It’s a plural pronoun being used to seemingly describe the singular Kendra? I checked the English story which has ’ Kendra and her friend have been on a wrestling team for about fifteen years.’ Which makes think it should almost certainly be ‘ケンドラと彼女の友達’ rather than ‘ケンドラと彼らの友達’ in this case.
I haven’t read the mini stories in a while, but they seem grammatically correct though somewhat unnatural. For this lesson, the use of 彼ら is pretty excessive and pretty unnatural IMHO, but it’s about showing you how 彼ら can be used I suppose. Japanese pronouns work differently than English pronouns.
I’ll caution I’m not an expert at Japanese Grammer, but when I read it, it sounds like it’s Kendra and her friends rather than Kendra and her friend (singular). The plurality of the pronoun extends to friend to make definitively plural.
I imagine it’s grammatically correct but sounds pretty unnatural to me, or maybe it’s just pretty formal way of speaking. Unlike English, in Japanese you use pronouns far less, instead referring to people by their names. Usually, you hear something more like ケンドラたち instead, which is Kendra and Kendra’s group of (presumably friends). Or, the pronoun would just be left off all together.
= Kendra and their friends.
I read the lesson, and it appears to be an error. Looking at the Spanish mini-story it refers to just one other friend, and not a group like “Kendra and their friends” would suggest. It appears they want to say ケンドラと彼女の友達, as you said.
I think you can read these mini-stories to see how the sentences fit together, but as these are translated to be as close to the English mini-stories as possible they don’t always sound natural. Your future input based on untranslated Japanese content will give you more natural sounding expressions.
英語のミニストーリーでは
“Kendra and her friend have been on a wrestling team for about fifteen years.”
となっています。
だから「ケンドラ と 彼女 の 友達 は 15 年間 レスリング の チーム に 所属 しています 。」が正しいです。
でも、全体的に自然な日本語ではありません。
「ケンドラは友達と一緒にレスリングのチームに15年間ぐらい所属しています。」のほうが自然です。
（～と一緒に＝with ~）
（～ぐらい＝about ~ ）
「所属しています」は「入っています」と言うこともできます。
この文がフォーマルかもしれないというのは違います。英語の授業で生徒が翻訳するような（下手な）文です。
私は日本人ですが、「彼」「彼女」「彼ら」という言葉を使うのは英語を翻訳するときだけです。
In the English Mini Stories, it says, “Kendra and her friend have been on a wrestling team for about fifteen years.”
So, “ケンドラ と 彼女 の 友達 は 15 年間 レスリング の チーム に 所属 しています 。” is correct.
However, this is not very natural Japanese overall. “ケンドラは友達と一緒にレスリングのチームに15年間ぐらい所属しています。” sounds more natural.
(～と一緒に＝with ~)
(～ぐらい = about ~)
“所属しています” can also be replaced with “入っています.”
The idea that this sentence might be formal is incorrect. It’s more like a (poor) sentence that students would translate in an English class.
As a Japanese person, I use words like “彼,” “彼女,” and “彼ら” only when translating from English.