Does she get travel sick? If so will she need some medication or travel bands to help? Or even ginger to help.
Does she have a particular teabag or drink that she starts the day with? Might want to take a few teabags with you or some favourite biscuits etc to have a take with you as a taste of home, particularly if she is ever likely to have hypos and need a quick sugar fix.
Some sucky sweets to the airplane to suck on take off and landing.
To go with the adapter get a multi plug and one with usb ports in too so you can charge several things at once.
A money belt so you can split your cash up.
As well as a guide book do some research beforehand to see if there any things that you would like to do and see if you can get advance tickets more cheaply. Also download maps onto your phone do they are on there and you don’t need to rely on data for it (regardless of cost, the moment you need it is going to be down a back street with rubbish reception!). Also see if there are any good tourist guide apps to go with the book so you have both options.
Also google tourist scams in Venice so you can be aware in advance - you hear horror stories of people looking at menus that seem reasonable without realising that they are for indoors and if you want to eat outdoors then it’s 85 Euro’s per head or 50 euros for a coffee and ice cream in st marks square etc. Forewarned is forearmed!
Does she have any other relatives or friends that might want to give her a present that would be something to do in or to do with Venice? They (so long as they can be trusted not to spill the beans!) could give her a card and just say present later or with OP etc - then give you a card and euros to say ‘your present is a trip to Murano and Burano and the money to buy a glass necklace as a momento of your trip’ for example. Or a voucher for a photo book to make up for memories of the trip etc.
If either of you have a camera rather a camera on your phone - a spare memory card as I bet you’ll take loads of pictures. If you just have phones then do you have a way of backing them up while you are out there? You can get memory sticks that you can use that again won’t use data to transfer images.
A small light day bag to put things in when you buy things out there, or want to put your coats somewhere safe rather than in your hand if it’s a nice day and you don’t want to wear them all day. (I have a couple of ‘sue’ shoppers from a company called Mia tui that are only £4-5 but hold loads and also have a zip section and fold down very small and are very light. They also have a slightly bigger stronger travel bag that folds small but has a slip on it so it will go on the handle of a wheelie bag.
Would she enjoy a novel set in Venice to read as light relief while out there?