Each jurisdiction will have its own standard operating procedure for handling and post processing of photographs (whether taken by a police officer, the general public or CCTV security cameras). As long as the steps are followed, the photos will be admitted as evidence. If there are claims that the photo has been tampered with, the officer that took the photo will usually be required to testify that it is the photo they took. In the case of UV and IR forensic photographs, they might also have to explain any steps they've taken. The jury will weigh up the credibility of the police officer in coming to their verdict. In itself, nothing hard, but photographic evidence is obviously very damaging to a defendent, so they'll argue any way possible to have it excluded. The data security kit makes it a little harder to raise objections as it is one less step to overcome. (Yes - I watch a lot of CSI and Law & Order)What does this mean: "Oh, and doesn't Canon have some kind of strong signing of an image that it comes from a person/camera? Maybe he should look into that, since it probably would help with chain of custody kind of thing for any evidence." Strong signing?Canon offers a Data Security Kit for some cameras, although a quick google suggests it's been cracked.
If you friend wants something discreet, try a micro four thirds camera with a 100-300 lens. The Nikon V1 with a 70-300 would also be pretty competent in daylight - That gives you about 810mm in real lens talk.