Hi, I'm Pete. Welcome to JustAnswer! I'm reviewing your question now, and will post back with your reply ASAP.
The Canon Rebel 3Ti EF lenses will work with pretty much any other Canon DSLRs with sub-frame sensors - e.g. XS, Rebel XSi, Rebel T1i, T2i, T3, EOS 50D, 60D, and EOS 7D).
You are correct that there is not much difference between 18MP and 23MP in terms of quality - the main benefits she might get in a new 23MP camera are better functions on the camera itself, rather than more detail on the photograph.
If you want a really serious 50MP camera then you would be looking for the EOS 5DS or similar:
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Yes, I agree it would definitely be worth going for double the MP for just an extra $1000.
Yes, the EF lenses she already owns are compatible with other Canon DSLR cameras including the 50MP camera.