Hi there;
Here's the solution I would offer you:
Yes, you need to do a clean install...
1. Find the COA on her machine; not only does it have the Key on it but, it will, also, state which version of 7 it is.
2. Take the HDD out & hook it up to another machine. She must be able to tell you what data/files she wants/needs to keep. Usually it's Photos & other stuff in (My) Documents, @ least & maybe more, as well. Save whatever to that machine (to retrive later) or onto disc or flash drive.
3. Put her HDD back in & do a clean install... that can be done from your disc but, you use her Key, from (again) that sticker on her machine. It's the Key that is important not, the disc. All Windows 7 SKUs are on the OS discs... it's the Key that determeines what you get installed.
The reason, I suspect, it won't start, even in Safe, is corruption in the registry files & or the Boot is broken or missing. Hence the need to clean install.
Once you have the OS rebuilt, put in MSE & all the Windows Updates. Then put back the data/files which, were saved.
I trust this helps.
Cheers,
Drew