We've all had that one friend whose tab is perpetually growing. Venmo requests for a split group tab always seem to get overlooked or perhaps they do a symbolic reach for their wallet when the server places the bill on the table — but no money actually ever materializes.

If you're not particularly good at confronting people, this type of friend can cost you a significant amount of money.

Harvard-trained etiquette expert Sara Jane Ho says that doesn't have to be the case. Ho is the founder of the finishing school Institute Sarita, host of the Netflix show "Mind Your Manners," and author of an upcoming book, also called "Mind Your Manners."

"You can send them friendly reminders," about paying you back, she told CNBC Make It.

But if those don't work, you might have to just consider it a learning opportunity.

'Lend as much as you're willing to lose'

Let's say your friend hasn't fulfilled the Venmo request you sent yesterday. Give it a little time, Ho says. There's a chance they are planning to get to it. If it's been a week, though, you can nudge them through another medium like with a text.

"If you want to confront a friend who never pays you back, you can either send them friendly reminders via email or in person saying, 'By the way, do you have the $100 that I lent you last week?'" she says.

If they continue to ignore your requests for months, it's safe to say they aren't planning to pay you back and you'll have to eat the cost. And while you might have lost money, you hopefully gained some useful knowledge about this person going forward.

"My father always used to tell me two things. Firstly, "neither borrower nor a lender be,"" she says.

This, from Shakespeare's Hamlet, means don't put yourself in the position of loaning money to friends or borrowing money from them and indebting yourself to someone else.

"Secondly," Ho says. "Only lend as much as you're willing to lose. If they really don't want to pay you back, then you've kind of learned a lesson."

If you know it's unlikely that someone will pay you back, limit your hangouts to free or budget-friendly activities. Instead of grabbing dinner, get a coffee and go for a walk.

And manage your expectations: If you buy a round of drinks at the bar, don't expect that they'll do the same.

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