Negotiating your grief along with everyone else's grief can be tough. Your Dad must have found seeing her page really upsetting, while you found it helpful. I'd be honest and let him know it upset you to have it removed so he understands he needs to talk to you before decisions are made. It might be a good time to discuss any other memories you have and perhaps division of some of your Mum's belongings so you have some things with you that are meaningful to you but may be overlooked by someone else?