OP, she could legitimately reduce her insurance by putting you, a friend, DP, sibling or parent on the insurance and it will probably be cheaper unless they have a terrible driving record. I think the rationale is that driving will be shared instead of a tired driver driving.
Both DP and I are mid 40s with good driving records. I have a clean licence and DP has 3 speeding points, so he is arguably a higher risk than me. On my own, my insurance is £290, but it was £230 with DP on it. DP probably drives my car a handful of times a year. I am also on my sister's insurance, who is a single parent and it does make her's cheaper as I have been driving a lot longer than she has. We do sometimes go on days out together in her car a few times a year, and sometimes I do share the driving with her, so it's not like we only do it to get cheaper insurance - there is an idenfied need.
If your friend hasn't yet bought a car, another thing she can do to get cheaper insurance is get the lowest group car possible - something like a Skoda Citigo, Seat Mio or VW Up, Peugeot 106 etc are very low group insurance (1 or 2 depending on model). It might even be worth spending more on a car like that than getting a much cheaper old car, that is higher group so more expensvie to insure.