I also feel strongly that you should gently bring it up with her mom, if for no other reason than to encourage her to keep an eye on her daughter. Her daughter is just a child and is innocent in what she's doing, but predators or potential predators might use her actions as an "excuse" for abuse. All I'm suggesting is that mom pay extra special occaision when she is around other adults or older kids with her dd and make sure if dd is running around humping everyone's legs, her mom is right there re-directing her and just kinda watching to make sure no one takes advantage of it.
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And while I know some level of masturbation is normal at that age, constant humping and working so hard at it constantly definitely raises red flags for me. Just encourage mom to talk to her daughter and watch her daughter and think about who has access to her, just to rule out any situation where maybe a trusted person is being inappropriate (adult or older child). Who does her daughter play with? Does mom have child care and what's the set up?
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It's not about being alarmist, but it is about realizing that it CAN be a trouble sign, just like it can also be a slightly extended version of "normal". It's always best to just open up communication and in a respectful, supportive way, flag it for mom.