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I would strongly recommend the 100-400L IS Canon lens for Africa. There are many times when 400 is essential for really strong impact shots. I have such a lens, and I have been to Africa with it. It's heavy, and somewhat cumbersome, but it's an excellent lens.
As a rule of thumb, the shutter speed should be at least the reciprocal of the focal length being used. 300mm, 1/300sec. However, remember that with a 350D, there is a 1.6x factor, so 300mm fl is the equivalent of 480mm, so 1/500sec would be the slowest practical use for hand held. (That said, using the IS gives 1-2 stops extra, so you would probably still get by with 1/300 sec)
Yes, you can switch IS off. With my 100-400IS lens, it is recommended to switch IS off when using it on a tripod, or on B settings. It comes complete with a soft case, and a lens hood, as well as a tripod attachment, which is very useful for carrying the camera around when the lens is attached, as it is at the point of balance, and I find it very easy to hook my fingers around it for convenient and comfortable carrying.
For astro use, the focal length can be securely set by tightening the zoom ring, so you don't get any creep. I haven't tried to use it for serious Astro use, so I cannot comment on actual results.
If you do decide to get the lens, (it isn't cheap), I would strongly recommend getting a 12 and/or a 16mm extension ring, which is very useful for taking "close-ups" of small birds and the like.
If your wife is going to S. Africa or Kenya, in particular, she should be VERY careful about carrying the lens and camera around in full view when in large towns, or anywhere where there are crowds. In game parks or country areas, it's generally OK, but warn her not to carry a camera around her neck when walking alone in the street or on the beach, as that is asking for trouble. Make sure it is covered by her travel insurance or household insurance for all risks, and take a note of the serial number. Sorry to be alarmist, but trust me, it's a fact of life.
I hope she has a wonderful time, and if you have any questions, feel free to PM me.
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