A question about metronome

[Gill the Piano]

The antique style wooden jobbies from antique shops/eBay are lovely, but you need to make sure they're calibrated properly, or they sound like a heartbeat rather than a clock! Safest is to buy a new one, and it depends on whether she wants to take it around with her (to piano lessons, say, or practise outside the home) or if it's going to live on the piano. The electronic ones come in all sizes from about 5" tall to weeny ones you can put in your handbag, not much bigger than a credit card. There's even one that fits in the ear! You can get ones which just indicate the beat, or ones which beep every 2nd/3rd/4th/5th/6th beat (ie, tell you where the beginning of the bars are). Some of them give you concert pitch A should you need to know that, but generally speaking that's a feature for string players who need to set the tuning of their instruments each time they play.
Your safest bet would be either to get a voucher from the music shop, or to ask her piano teacher on the quiet which one s/he recommends. Or make sure that the music shop you buy the metronome from will let you return/swap it if need be. Good luck!