I took the Eurostar over to London for five nights over the New Year holiday. I also like night shooting, and that's mainly what I did. As the previous poster said, the river area is the best bet, and Saint Pauls cathederal as well as Westminster Abbey are both nice. I was a bit dissapointed with the lighting though, as many significant buildings were poorly lit. The US embassy has to be the ugliest building in the city, if not the whole country, so she won't miss not being able to snap that.
In regards to buying, that's tough, but certainly don't get any photo gear. Haha. I brought back almost 40 pounds(the weight, not money) of brown rice, whole wheat flours, and scottish oats, and good peanut butter, because I can't find suitable ones here. Since you have a Wal-Mart nearby, that's probably not what you're in the market for. My mom though bought some nice tea accesories, maybe a pot and strainer or something. The shop was having a huge sale then, as were most stores, but those might be over now. There are some nice old bookshops on Charing Cross road if that is her/your thing.
I will try to find a picture or two to post. Oh, and definitely bring the tripod.
Canon 80D and kit zooms. Also 50mm 1.8 and 10-16mm.