To double it safely i will build a food business which most people really like.
I'll build a restaurant spending all $100k. Although doubling it will require some time but
it will be safe in that way. Rather than investing it on some networking or use it as trading capital
which have a high risk of doubling money.
This is one of the best move you can have with your $100,000. Since restaurant is one of the great business you can have in real life with money generating daily, and come up with double your money in the time will come.
I hope he knows how restaurants work.
In my neighbourhood some crazy lady who never had anything to do with that kind of business opened a restaurant, put all her savings in it and failed completely.
Now she lives from social welfare.
A restaurant is not always a safe bet.