Yes, I think a chat with an accountant or tax advisor would be worthwhile. Sums involved are substantial so you don't want to miss any possible reliefs.
I presume you had a look at the Best Buys here on Askaboutmoney. Takes in State Savings products as well as bank accounts. In the absence of background information, I'm going to ignore other options, e.g. pay down debt, give gifts to avail of tax-free gifting, making a pension contribution etc. Just looking at the question as asked. If she wants the potential for a return greater than deposit rates, she has to accept some level of increased risk. Doesn't necessarily have to be high risk. Could be with a portion of the overall sum - the bit she's unlikely to spend in the next 10 or 20 years etc.