I had a mobile at 10 - however, as it was over a decade ago, I didn't have any apps etc, as they weren't a thing yet really. It didn't have much storage, and a rubbish camera, but as it was a flip phone it was much sturdier than the phones you get now so not much risk of me breaking it. And I doubt it was that expensive whereas I bet hers is probably £100+. I only ever got to take it if I was going somewhere without my parents eg town with my friends. It's clear that my parents gave me it so I could contact them when I was by myself somewhere, but in this case it seems to have been bought mainly for entertainment purposes.
Also, I'm a guide leader and while some girls do bring phones to guides, they tend to be older- approx 12 years and up. I don't think any of the 10 year olds do, although it is possible they have phones but don't bring them.
Is there any other way she can access the internet though? Even for a limited amount of time? Because it is very common for young people (I think especially girls) to access entertainment online eg youtube, or dress up games etc. I know some of the girls at guides do, because I've heard them talking about it, but I couldn't tell you off the top of my head how old they all are. Also whatsapp group chats are common- could her friends have one she's in? Perhaps if she doesn't have any access to the internet at your house, she feels different to her friends and slightly left out there? Maybe she's made a couple of off the cuff comments about that when she's at her dad's, which may be partly why he's thought of this. Though, I suspect you would have heard about it before now from her if she did feel that way!
However, regardless of his reasons/intentions it does seem like he's attempting to exert some form of control in your house, and is trying to play you off against each other. I think it would be reasonable enough to say she isn't allowed to use it at yours- your house, your rules. And even if you do lock it away, make sure you put your number in it, and make a note of hers, so you've got it if you want/need it - I'm sure you already thought of that though 