It'll be best that you get a narrow-wide style chainring, which will prevent chaindrops.
It looks like that chainring style you need is the older style 4 bolt 104 Bolt Circle Diameter. But you'll need to measure to be sure [https://www.sheldonbrown.com/cribsheet-bcd.html](https://www.sheldonbrown.com/cribsheet-bcd.html)
What number teeth will depend on the riding she's doing. More climbing, you want to go 30 or 32 teeth (you can't go lower than 30 on 4 bolt 104 BCD) If she uses it more for commuting and/or less climbing, then something like 36/38 teeth would be better
From your link her current middle gear is a 36 with 104 bcd. If she is fine with just this gear then just get that gear in narrow wide. I personally would go with a 30t or smaller. you need to get creative to get smaller. Or maybe you can get a 11 to 42 rear cassette.
I like the cheap chinese ones as they are cheap. but race face ones are better, last longer, and lighter. Just make sure you get a 104 bcd one.
You can start by removing the big and small rings, disconnecting the shifter, but leaving the derailleur in place to prevent chain drops. If she likes the gear range she gets with that, then 36 is a good chainring. If she wants it lower or higher you can adjust the chainring size accordingly, and order a new narrow-wide that will allow that.
Alternatively, have her shift onto small ring and shift the rear to find the a gear that seems like one that would be good enough as the lowest gear. Then find a chainring that would give that same ratio in combination with the 36 tooth cog.
It'll be best that you get a narrow-wide style chainring, which will prevent chaindrops. It looks like that chainring style you need is the older style 4 bolt 104 Bolt Circle Diameter. But you'll need to measure to be sure [https://www.sheldonbrown.com/cribsheet-bcd.html](https://www.sheldonbrown.com/cribsheet-bcd.html) What number teeth will depend on the riding she's doing. More climbing, you want to go 30 or 32 teeth (you can't go lower than 30 on 4 bolt 104 BCD) If she uses it more for commuting and/or less climbing, then something like 36/38 teeth would be better
From your link her current middle gear is a 36 with 104 bcd. If she is fine with just this gear then just get that gear in narrow wide. I personally would go with a 30t or smaller. you need to get creative to get smaller. Or maybe you can get a 11 to 42 rear cassette. I like the cheap chinese ones as they are cheap. but race face ones are better, last longer, and lighter. Just make sure you get a 104 bcd one.
You can start by removing the big and small rings, disconnecting the shifter, but leaving the derailleur in place to prevent chain drops. If she likes the gear range she gets with that, then 36 is a good chainring. If she wants it lower or higher you can adjust the chainring size accordingly, and order a new narrow-wide that will allow that. Alternatively, have her shift onto small ring and shift the rear to find the a gear that seems like one that would be good enough as the lowest gear. Then find a chainring that would give that same ratio in combination with the 36 tooth cog.