S
Sunseeker
Guest
Numbers are so telling in 5e, it would probably be advisable to have a party - have the lone player play a couple characters and have a DMPC tag along. Or go really old-school and let the PC have a number of henchmen and hirelings.
I haven't found it to be a problem with one character and one DMPC (which is built as a normal character, no special DM-powers). I have not noticed a problem with running monsters straight from the book at the appropriate challenge rating, in fact it's a little easy.