Bit more detail on the car would be nice, ie year model etc.
The coolant if mixed correctly or possibly a bit high on the concentration can feel slippery between finger and thumb and this is normal. If there was oil in it you would see a slight film of oil on top. A lot of oil would show as a visible layer on top and be obvious because oil and water do not mix.
I've bought several secondhand cars over the years and have never used this 'test' so I think he's trying one on.