Considering it is a prebuilt check the layout of the current case.. many times they have upside down motherboards so a standard motherboard will not fit and you will need a new case.
i5-6600k would be the normal suggestion
motherboard will depend on case you will need to make sure it fits. I personally like ASRock boards and it looks like this one will work with her current ram
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157652&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-PCPartPicker,%20LLC-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
Also depends if she would ever want to get like an m.2 drive or something like that you would need to look for something that supports that. Also if you care about color scheme at all might want to look into it further.