Main use will be shooting at parties
I already have a number of spare Canon lenses and was thinking of donating my 24-105L to her and thus limiting the financial outlay to 'body only'. It would also have better IQ than any of the 100D/700D kit lenses.
Yes, the 24-105L would have better IQ wide open than the 18-55, although not as much better as you might be thinking. Not sure if 100D + 18-135mm STM kits are available where you are (they aren't in the USA but since you call it the 100D and refer to IXUS P&S, I infer that you're not in the USA) - that kit lens actually delivers better IQ at the long end than the 24-105L, even giving the 24-105L the benefit of stopping down to f/5.6 (TDP comparison).
The real issue with the 24-105L is that on APS-C it gives a FF equivalent of 38mm. Not sure which IXUS she has, but most of the recent ones start at 28mm or even 24mm FFeq…I'm not sure she'd be happy giving up the wide angle, particularly since you mention indoor use.
So, my vote goes to a 100D body (for the small size) and either the 18-135mm STM lens, the 18-55mm STM lens for cost savings, or if you want to splurge and one-up the Joneses, the EF-S 17-55mm f/2.8 IS which gives better low light performance and IMO is the best general purpose zoom for APS-C.
I was thinking almost exactly what neuro said except I didn't consider the 17-55 upgrade. I also agree with K13X5C. Get her something new that is hers and let her show it off and get a real thrill out of using it. And help her with it often so you can be her resident expert. Compliment her on her work.
Another +1 for Neuro's suggestion. I'd also like to second the "EOS M" suggestion as it was the very first thing that popped in my head. Buying the "full kit" of the EOS M + 22/2 (for low light) + 18-55 + 90EX flash sounds PERFECT for her uses, IMO. And if you're in the US can be had for $400. That's a LOT of bang for the buck as the IQ from both of those lenses is OUTSTANDING, the kit is compact and the entire package is quite versatile. If the 40mm f/2.8 STM is desired, it can be added along with the adapter for less than $250.