What you get from an ISA is that you don’t pay tax on the interest.
So it depends on whether you have enough savings to go above your tax-free allowance for interest payments.
If you're a basic rate tax payer, you can earn £1000 in interest before you have to pay tax on it. So if you have less than £20k in a 5% interest bank account, you're fine. Above that allowance you pay tax on the interest at your marginal rate. Ie if a basic rate had £40k in a 5% interest bank account, she would pay a total of £200 tax. She's still getting 5% interest on the first £20k of savings, but she's effectively only getting 4% interest on the other £20k savings. The other 1% (£200) goes in tax.
It's more of an issue if you're a higher rate tax payer or have more savings.
If you're a higher rate tax payer, the allowance goes down to £500. So if you have more than £10k in a 5% interest bank account, then you would start paying 40% tax on the interest for all your savings above that. Which would bring the effective interest rate down to 3% for your savings above £10k (60% of the 5% interest)
In an ISA, you don't pay tax on the interest, no matter how much savings you've built up.
So if she only has a couple of thousand, then no need to put it in an ISA. If she's expecting to build it up quickly over the next few years, then she might want to think about timing to put it into an ISA since you're only allowed to put in a maximum of £20k per tax year.