There actually is something in the first half that if you read a little between the lines does seem like it might indicate some negligence by the other girl's mother:
>> Kaye thought it was cute and snapped a photo with her phone and texted it to Eme’s mom, who’s a friend of hers. She replied that if at some point the girls wanted to ride Eme’s sister’s new electric bike (it was a bat mitzvah gift), they were welcome to walk over and use it.
It was a new electric bike, and it was a gift for the sister, and the sister was probably 12 or 13 since that is the age for a bat mitzvah. I don't have any siblings, but from what I saw of friends who did growing up, a 12 or 13 year old who gets a gift like this is going to monopolize it for a while.
I'd expect then that Eme had little to no experience riding it, which Eme's mother should have known. At the very least that means they should not have been allowed to ride together, since riding with a passenger is much harder than riding solo.
Eme's mother than should not have invited the girls over to ride it, unless she was going to supervise to make sure they didn't do anything too risky for their skill level. If she did not supervise that might be negligent. If she did supervise but allowed them ride tandem that might be negligent.
The second half, I completely flipped. The author doesn't specifically claim it in this case, but it seems pretty likely that the brakes failed, and that the brakes are either undersized or low quality on these bikes. I haven't owned an e-bike before but the idea that you need to do brake maintenance every 200 miles is totally unreasonable. I've owned owned several bikes and ridden them way more than 200 miles and I don't think I've ever had to maintain the brakes at all.
I had been thinking about buying one of these actually, and now I will steer clear.