I think the combination of the 18-135 and the D40 would be a nice setup. I don't think the weight would be a significant factor unless she has problems with her hands, such as rheumatoid or severe osteoarthritis, or if she has a tremor.
There is a weight difference from a P&S model with everything compact and in one smaller body. Has she told you what she might like? Has she ever held a D40?
The kit lens that usually comes with the D40 is the 18-55. Much smaller and lighter than the 18-135, but more limiting. The 18-135 will focus quickly and give her a longer reach, so she can shoot from farther away, so to speak.
As far as lens compatibility goes, this can be confusing. The general lenses that she may need will autofocus on the D40. By this, I mean the following lenses: 18-55, 18-70, 18-135, 18-200VR, 70-300VR, 17-55, and 70-200VR, among others.
The one lens you may read about that doesn't autofocus on this camera body is the Nikon 50mm 1.8. This lens does not have a built in autofocus motor. It can be used for manual focus on the D40, but she may not want to do that. It's not difficult, however.
Lots of older Nikon lenses do not autofocus on a Nikon body that does not come with a built in focus motor (D40 and D40x). The D50, D70, and D80, as well as the pro models, have built in focus motors in the body. There must be a focus motor either in the lens or the body. The lenses listed as compatible with the D40 series by Nikon are:
Compatible Lenses*1: Nikon F mount with AF coupling and AF contacts Type G or D AF Nikkor
1) AF-S, AF-I: All functions supported
2) Other Type G or D AF Nikkor: All functions supported except autofocus
3) PC Micro-Nikkor 85mm f/2.8D: Can only be used in mode M; all other functions supported except autofocus
4) Other AF Nikkor*1/AI-P Nikkor: All functions supported except autofocus and 3D Color Matrix Metering II
5) Non-CPU: Can be used in mode M, but exposure meter does not function; electronic range finder can be used if maximum aperture is f/5.6 or faster
6) IX Nikkor lenses cannot be used
1. Excluding lenses for F3AF
For your purposes, I think the D40 and the newer Nikon lenses would be fine.