While your friend might have some valid points for
some kids, my personal experience has been the exact opposite in almost every single instance. While teaching the music, you can also be teaching the child reading concepts. It's not a difficult thing to do, doesn't take any more time than an a lesson would if the child could already read. They just sort of incorporate themselves. It's actually a pretty fluid process. It could very well be exactly what she needs to give her that little boost of desire for reading. You just never know.
I've taught children how to play various instruments for years-piano, clarinet, flute, guitar and trombone are pretty much my primaries(though I can play others and can teach
some on them as well, I'm not as good as I'd like to be, lol). I have taught children as young as 3 right on up to adulthood. My experience is that more often than not the younger they begin, the smoother it goes. They tend to absorb so much more at a younger age than an older one. They catch on faster, they remember far more of what they're taught than someone who has fifty million other things to think about. Most people don't learn to play an instrument-if they learn one at all-until roughly middle school to high school ages. There's a huge gap between the prime learning age for basic music concepts and the age when most begin to learn them. Basic music concepts, even if not taught while learning an instrument, really should be taught pretty early on. Not that they cant be taught later of course.
You don't actually need to know how to read to play an instrument. Most will need to know how to read to play really well, but it's not an all or nothing sort of deal. There are a LOT of people who learned to play an instrument long before they learned their alphabet. Some of the greatest composers ever didn't exactly have the best reading and writing skills in the world going into it-or even later in life for that matter. Just look at how many blind composers and musicians we've had over time, as well, who likely didn't have the best resources for learning what they were playing or writing actually looked like(long before most resources we have today ever existed, of course).
I'm not fond of people who poo-poo younger children learning an instrument. I've rarely found they have a good reason for it. What they usually give is a reason(or reasons) they think sound good, but in practice really aren't the truth at all. So I'm probably extremely biased. But most kids who DO have issues sticking with an instrument, or even learning it, in my experience, are the ones being pushed into doing it in the first place-no matter the age. The heart just isn't there. Sure some kids eventually decide they don't want to play whatever instrument they chose, but at least while they are learning it they're right there with you.
I say if she wants to, you're willing to shell out the money(because it's not always a cheap hobby, lol), you should pursue it. It can't hurt to at least try. If it doesn't work out, then it doesn't. It doesn't mean you, or she, failed. It just wasn't meant to be, imo. But if it does work out, just think of all the benefits to her. I think it's worth it, personally. Though it is something every family has to determine for themselves.
My kids know how to play piano, they'll be moving on to guitar next, or whatever other instrument they decide. I let them lead the way. I was quite happy when they showed interest. Though music is a huge part of our lives, I never wanted to push playing one on them. I wanted them to decide. They did, on their own terms, and they're really loving it. Ds started playing long before he could actually read too and he's never had very many issues. Even when he has the attention span of a grain of rice, his inability to read properly has never been a problem or even contributed to it.