Ive bought and sold a few
meet up
suss him out
take a general tour of the round
work it together
meet as many customers as possible even if it means going back in the evening together
when happy with all of the above hand dosh over
when working it together he should be telling people that your just helping him for a while as too much work on or just got a big contract .... NEVER USE THE WORDS TO A CUSTOMER SUCH AS BUYING / SELLING THE ROUND.
also drop a letter into customers you not met when you start working with his and your full contact details explain that your doing the windows due to him being too busy etc.