Honestly, I would just consider keeping it in the property. If the management is too much hassle then consider getting an agent to deal with that.
Transferring his assets to stocks needs very careful consideration.
Firstly, there is likely to be a big CGT hit if he liquidates his property.
But much more importantly, stocks are more volatile and liquid than property, so every second of every day it's possible (sometimes unavoidable) to see the changes in the market and know how you wealth is going up and down as a result. I feel this is psychologically a big point against financial assets, it can turn people into short term gamblers and the constant rollercoaster is not good for a person's psyche.
For many people it's like if you have £20 on a horse in the 3:30 at Cheltenham.. you then have a vested interest in the outcome, and it triggers a flight or fight response somewhere deep in the brain.
Probably one of the reasons why he invested in property in the first place is that it probably felt safer than stocks (and this was a good decision in hindsight).
Why change what has worked well for him? He would be taking big risks in changing his retirement nestegg into something that he is much less familiar with.
You mention that he has cash as well as property - by all means put this into the stock market, but if it were me in that situation I would keep the real estate.
