100bitcoins and a well settled busness will be more than enough to get retired after 20years but i think 100bitcoin is little bit harder to achive in this time but you can try
It may be more than enough, but it depends on how the currency will rise in value over the years.
Depend on his need to wag money.so you can not say 100 bitcoin will be enough .if you looking to retire after 20 year than why not to invest 1 btc in trustable mining service which give you good profit.
I don't know why you're recommending for him to put some money into a mining service. Investing in cloud mining is literally betting against the network and assuming that the overall hashing power is going to go down over the time that you have your contract, and that's already an unlikely bet. On top of the fact that a majority of mining services turn out to be scams in one way or the other.
I did some calculation on the cloud mining. If there is no difficulty rise, you can make some profit. Otherwise, there is no profit.