This is a common thing to feel after you hit a bet.
Of course whenever you win it doesn't feel like you bet enough. But you have to realise that there is some probability that the bet would not have hit, and you would have mitigated some losses due to you betting a smaller amount. This is especially true when it comes to parlays which are generally very -EV (as each leg is -EV and it is multiplicative).
Take the winnings and don't feel the need to chase the imaginary losses that you are keeping track of.
Absolutely agree with you. No one knows what would have happened if this guy had bet $10 instead of $1. It's ridiculous to think that the winnings would have increased tenfold. He could have lost that $10.
But this friend's behavior is something to think about. If someone is willing to lose a friendship for $10, then that relationship can't even be called a friendship. I would stop communicating with such a person as soon as possible.