Sky broadband needs a BT telephone line anyway as that's how they provide their service. If all she wants is Broadband then there are any number of suppliers out there. All she needs to do is decide how much she wants to pay and how often she will be on line.
The only advantage Sky can offer is a package deal that includes TV, telephone and broadband. If she wants Sky digital TV that might be the way to go but Freeview has a pile of stuff with the advantage that it's free.
She wouldn't need a 'package' if she decides to subscribe to Sky TV. She can just add broadband, either free or £5 per month. I have the fiver a month broadband and am very happy with it, not had any problems. My telephone account is still with BT.
My daughter is moving. She has a laptop. The house has a BT line and Sky TV. What would her best option be to connect to internet?
I haven't found anything better than the Post Office home phone/broadband (up to 8 meg) package at 19.53 per month including line rental AND free evening and weekend calls + international and mobiles. Unbeatable in my area.