@jorritdv , I speak from the perspective of somebody who's just being trying to give his own son ideas for Christmas!
Whether for the hiking trail or the Camino - which are very different - overall I have to agree with
@Robo on this. If as you say she's already got all the basics, then she has everything she needs. Many of us carry a luxury item or two, but those items are generally very individual. They also depend a little on the trail being walked - for example a number of members on here carry a water heating coil and a cup on some of the lesser travelled camino's. Not something you need on the Frances or the Portuguese! The only thing I can think of that she may use on the camino but is unlikely to have for hiking is a super light mini daypack. On camino you're staying every day in accommodation so your pack stays there and it's useful to have a super light day pack for wandering around town, shopping etc. Mine is 18l, weighs 120g.
Most of us are more likely to shed Gear rather than add something new -
remember wether on trail or on Camino we all want to keep the weight of our packs down as much as possible.
However....
Hiking trails are somewhat different to the Camino. (Assuming you're not just talking about day/ weekend hiking, but actually going off for a week or more and camping. I've never hiked in the Netherlands so I have no idea.)
Clothing remains much the same, but my trail pack is a lot larger than my camino Pack. On trail I need my tent and sleeping pad, plus a better sleeping bag, and a water filter. I also need cooking Gear. (Some only eat cold, I am not one of them) Has she ever mentioned that any of these items could be improved on or upgraded? For example she might be happy with her old stove but the new ones are much smaller and way lighter. The MSR Pocket rocket for example. They're not extremely expensive either. Ditto with the water filter - as great as the Sawyer mini is, the Sawyer squeeze is significantly better. If she's already got one consider getting her a gravity feed system for it. Light, but so, so practical!
Cup and cookpot - titanium is way lighter than aluminium.
Good luck