The picture doesn't suggest it's been outgrown ...Presuming she hasn't outgrown it. Service it and let her ride it. She'll never thank her parents for just selling it, no matter how shiny the replacement might be.
If she's too big for it despite adjustment, you could go shopping for other mixte framed bikes and see if she can be tempted away. But unless it's truly tiny for her, keep it running until she's ready to change.
Fine let her have an MTB just don't sell the old bike (nowt wrong with someone owning more than one bike, there are 12 here between my 10 and Maz's 2)I don't want to get rid of, this is my husbands choice . He says it will be too hard for her to ride the trails we will be taking due to its weight. she was riding a mtb in the store today and liked it. This after I dismantled her bike to get new wheels, tyres and saddle for her.