Either a down payment for a home in a decent area, or the purchase of stable non-depreciating assets. I'd be hesitant about long term bank investment in the current climate, but it's not a terrible option either.
The economy is in a bit of a mess right now, a sudden 100K will make a younger person feel rich until it's all down the toilet in many cases.
We're largely dumb arses until around 30 in my experience, Id not be surprised that should she nail the 100K the money would be spent on cars, gifts, and holidays.
I do feel that there's a bit of a disconnect between users, OCUK has always been quite heavily populated by well to do middle class types that rate the value of money very differently to others. I come from a (legitimate, not Maggie T style) working class background, there's a lot of factors that differ when looking at money. In the North East UK you could buy a two bedroom semi-detached in a so-so area for under 200K, obviously not everyone wants to move or deal with a drop in quality but I do think value factors into long term decisions. There's a ton of investment from big tech companies in NE UK right now, house prices are relatively low in some areas but I doubt that'll be the case in 10-20 years.
However, that money is hers to spend however she wishes, period.