Spiced vinegar.
Extra spices. You can get packets of 'pickling spice' but you can also buy loose spices, black,white and pink peppercorns, corriander seeds and other 'seeded' spices, not powder.
Some flavoured vinegar, onions pickled with pickling vinegar and a glud of raspberry vinegar are fabulous.
A book on pickling.
Any jars, jam jars, mint sauce jars, nutella jars etc. I once got a box full on freecycle, they were left on a door step so you could still do this in lockdown.
Sticky labels for the pickles or pen that writes on glass.
Salt and sugar, salt to dry out the veg before piockling, sugar in case she wants make chutneys.
Big bowl to put brine in over night.
Can you tell I make pickles?
I sterelise the jars and lids in the dishwasher. I have done the hot method @2beesornot2beesthatisthehoneybees mentined but I use smaller jars, mint sauce jars work well for this.
You can also, I believe, use a dishwasher instead of a pan for the hot method.
Vouchers seem a good idea.