If she is working solely online then she'd be opening up a serious can of worms if she tried to declare anything to the Turkish authorities. In my (totally unqualified) opinion, she is better having an address in the UK, such as an accountant, and keeping everything there - tax included. Depending on earnings, its likely she'd pay less tax in the UK as well.
When she gets here, she'd also better keep her mouth shut about what she is doing as there is always some jealous fcuker who will take great delight in grassing her up to the polis/maliye. Sad, but true.
JF