Will Double Deluxe install just Nightlife & Celebrations if the base game is already installed (or base game & University)? Or does she have to unintall the base game & then install Double Deluxe?
Although Double Deluxe is the combination of 3 packs, it installs like one pack. It is mainly intended for first-time buyers.
Personally I've never tried to install DD over the regular base game, so I wouldn't know if it's even possible or not. But I suggest that she backs up her saved game first, uninstalls the base game completely, and freshly installs DD, just to be on the safe side. Not worth risking bad things like registry mess-up. Possibly the uninstaller can be screwed up, making it impossible to uninstall and reinstall, which she will be forced to do someday.
If the second is the answer, won't it screw up the game when she installs University after Double Deluxe?
No. Unlike TS1, installation order does not matter in TS2. It's the only possible way to install Uni for DD owners, anyway.
I'd just have her torrent Nightlife & Celebrations, but she is a relative newbie and doesn't seem the kind who'd want pirated copies.
Well, I've acquired Double Deluxe even though I already have NL and Celebration, just to reduce a couple of steps in my seasonal Dance of the 20 Discs.
Edit: Ninja'ed by Gwill.