Your friend needs to find out how much data she'd get from a fiver a month. It can't be much!
https://www.vodafone.co.uk/shop/bundles-and-sims/sim-only-data-bundles/
Then she needs to decide whether she actually needs to get data via a mobile phone network (or whether wifi access only will suffice).
If she actually needs data from a mobile phone network she needs to work out how much she might need (using one of the many online calculators that are available) and decide whether or not her fiver's worth will be enough. If it's not, she then needs to find out whether her iPad is locked to the Vodafone network (I doubt it) and whether she can get a better deal elsewhere. (She almost certainly can. Vodafone is expensive).
If your friend is happy to connect to the internet solely via wifi though (using her home/office router or any of the countless free wifi services available in Wetherspoons, Costa, Tesco, Asda, etc,etc) then she doesn't need to have a SIM at all.