I know it's well-meaning, all these suggestions to help but most are frankly awful ideas & incredibly patronising.
Could you put some money in an envelope and put it anonymously through her letterbox late at night or when you know she'll be out? Maybe write "A gift for you, with love"
I would just be so stressed if I got random money. I couldn't decline it, I couldn't thank the person, I'd feel so uncertain about the whole thing.
Would you have chance to slip a £20 note(s) somewhere into a pile of paperwork or old handbag she might have around in her house,
Or this. It's intrusive & ofc when the person would find it they'd know they never had £20 notes randomly stuffed in places & again are back to wondering wtf
Ideas like paying for lunch or coffee, or treats that are otherwise a challenge is much better. If I knew someone could genuinely afford it when I couldn't, I'd gladly accept, within reason, and be appreciative. I'm not stoney broke but every penny is accounted for & going out for a nice meal isn't an option. Recently my brother treated me & my DD to a really nice meal. He is very well-off & I enjoyed it & appreciated his generosity. It was such a nice meal, the food was perfect & I enjoyed every mouthful. I have lots of practical things that cause me stress money-wise but the chance to do something nice without worrying was worth more to me than anything else.