Best advice I could give her (I'm a guitarist not a keyboard player, but the basics of rhythm are the same) is to buy a metronome and practice with it.

Carl Humpfries's Piano Handbook and Piano Improvisation Handbook are great, and cover enough for even an absolute beginner. I like noodling around with no previous musical knowledge, and they work very well for that. I think both include pretty decent sections on rhythms, and discuss pretty varied styles.

I play bass in a few bands, rhythm is tough but learnable. Easiest way to get good timing fast is:

1. Listen to the music you want to play first. If you only have the sheet music, Musescore is a fantastic software to hear sheet music.
2. Keeping the rhythm that your heard in mind, play what you want to play. Once you get it, it will feel right. If not, play the music VERY slowly, slow enough that you can do it quite easily. Set a metronome to this slow speed, then increase the BPM by 5. Find your limit, back off the BPM slightly, then practice at that speed until you can nail it.
3. I cannot stress this enough, record your playing! Knowing how you sound is so so so helpful, it lets you hear what your playing without the distraction of actually playing. Super helpful to see exactly how and where your out of time.

In F Droid, there's an app called Semitone that is a tuner, metronome, and simple keyboard. That might be a useful tool for them and others.

I'm an adult learning piano for a year now.

I hate playing to a metronome. I think it's boring. My internal clock isn't the best either, so counting rhythms ("1, 2, 3 and 4, etc") works when I'm trying to hear sheet music in my head, but it doesn't help me actually play in time.

The 2 things I prefer are either playing to YouTube backing tracks or a chord progression generator.

YouTube has thousands of either plain drum tracks or "band" tracks to play along to. For drum tracks, search for "rock drum track 100 bpm" and for instrumental tracks, something like "pop backing track in e flat major" and you'll get a list of things you can play over and keep in time.

I've also been using the musicca chord player to generate quick chord progressions to play along to. That link is fun because you can turn sounds on and off to keep it basic or more complex.

There are also straight rhythm exercises on YouTube as well.

As someone who tried for many years to learn guitar and bass, but never really got anywhere, with the piano I got a teacher after a few months. I have learned SO MUCH FASTER!

Learning on ones' own is possible, but think about it like having a list of ingredients, but not a recipe. You can grab bits of music knowledge, but not know how to put it all together. Plus since you don't know how to do it right from the start, how will you know if you're doing something wrong?

The teacher works like a personal trainer would at a gym. They don't just randomly say do this or do that. They let you focus just on doing the work at hand, while they're watching you, making sure you're working efficiently, accurately, and not just meandering around wasting your own time or doing things that are ineffective. It is so worth it.

After a year, I'm learning a 2 page song every week, while also doing a theory lesson, a bunch of scale exercises in rotation, working on sight reading and ear training, and learning improvisation skills. My teacher gives me all genres of music to learn so I learn multiple styles and techniques and I never get bored. I look forward to going every week and it's so worth the modest amount of money for a half hour lesson once a week.

I hope some of those things are useful to you.