I'll play devils advocate......technicially there is no legal requirement to be insured to work on clients, however, it's pretty amazing how fast relationships disolve when things go wrong. You may class her as a friend now but if she developed an allergy and couldn't work, some 'no win, no fee' lawyer tells her she has a claim against you for her loss of earnings and bang! she ain't a friend anymore! What if you spilled acetone on her favourite new £150 boots and ruin them, would she still be a friend then?
Personally I wouldn't touch a client without insurance in place but I'm fairly risk adverse. In the grand scheme of things Nail insurance is not expensive.
Lets do some guestimates - if she comes to you every 3 weeks for a year that's roughly 17 times. If you charge her £25 per rebalance that's £425 in takings, a years insurance is probably 10% of that. No brainer for me