Mark 1 had posted most of my reply already so I have just these two points to add.
When you ask about an unrated PPL flying as a safety pilot in IMC, its not relevant. If the commander of the aircraft is licenced to fly in IMC then there is no requirement for a safety pilot. During IMC training the instructor would be PIC and if ‘simulated instrument flight’ was undertaken they would maintain the lookout. A third person in the aircraft could act as an observer, if the commander so wished.
The other aspect of ‘safety pilot’ which is more of an undefined area, is when a pilot takes someone who has more experience or qualifications with them. This might happen if there were concerns with the weather or the difficulty of the flight. For example a low time IMC pilot, who is qualified to fly in the conditions might take someone with more instrument experience in case things get really hard. Sometimes these arrangements are quite informal, e.g ‘We’re off to XXXX, I’ve never been there before, I’d appreciate you coming with us’.
RobinHood
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Imagine a guy on his first IMC trip, in thick cloud, with a low cloud base. He has his PPL and a class rating, but could he seriously be regarded as being suitably qualified to act as PIC, should it become necessary???
See above. The scenario you describe is not legally possible. If someone has no IMC rating then they can only fly in IMC as a student with an instructor suitably qualified.