[bkfizz02]

Look, if your friend insists on adjusting the probability of a flush draw based on the idea that his opponent might have some of his outs, that his opponent has a given card, then just encourage him to do that calculation and compare it to the calculation you "claim" the answer is.

Example:

You: 2s 3s Him: unknown Board has 2 spades after 4 cards

To calculate the probability you make your flush on the next card, you simply take 9 outs out of 46 unknown

or
P(Os) = Probability opponent has 0 spades
P(1s) = Probability opponent has 1 spade
P(2s) = Probability opponent has 2 spades

P(f0) = Probability of making flush given opponent has no spades = (9/44)

P(f1) = Probability of making flush given opponent has 1 spade = (8/44)

P(f2) = Prob of making flush given opponent has 2 spades = (7/44)

So the probability you will make your flush is :

P(0s)*P(f0) + P(1s)*P(f1) + P(2s)*P(f2) = 9/46

note: This is not the probability you will win the hand
Clearly in this example if you have 23, and your opponent has 2 higher spades, you are out luck

If your friend can't figure out the probability his opponents hold 0, 1 or 2 spades given that he holds 2 spades, stop trying to explain this concept to him.

If he does this calculation, he'll be able to understand why your answer is correct...

Hope this helps