WATCH: Woman busts best friend cheating with her boyfriend

It’s the age-old question – how does your best friend act with your boyfriend when you’re not around?

One woman was so determined to find out the answer, she set up an experiment after getting bad vibes from her bestie.

Kelsey and her boyfriend Sam took part in the set-up for the To Catch A Cheater YouTube channel to see what would happen with her friend Britney.

“I feel like she cannot really be trusted," explained Kelsey. "She’s the kind of girl that likes somebody that’s already dating someone.”

The set-up saw Sam running into Britney at a coffee shop, with the conversation soon turning flirty.

From talking about nude pics to making plans to hang out without Kelsey, things really escalate when Sam puts his hand on Britney’s thigh… and then tells her it’s over with his girlfriend.

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“I really plan on breaking up with her," he tells Britney. "She’s not all that, we both know that.”

The pair then share a sneaky kiss (all caught on camera, of course) and no surprises here but Kelsey completely loses it.

The redhead calls Sam straight away who puts it back on his suspicious girlfriend for putting him in the situation to begin with.

“I thought this is what you wanted, you set this up. This is for us,” he says on the phone.

“I’m saying now you know what she’s all about… This is your fault.”

Watch the video above... and learn from Kelsey's mistakes.

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