Posting on behalf of my bf since his is not on reddit

My bf has a friend (22M) whose girlfriend (22F) started to work a few months ago, where she became bestfriends with her male co-worker pretty soon.

[To note – She never had any male friend before her boyfriend.]

She spends more time with her male bestfriend than with her boyfriend (due to office obviously), but also after office hours, she texts the guy alot.

Her boyfriend who is good friends with my bf showed him her chats (her boyfriend has her social media given by her). Well it didn't seem inappropriate as it was mostly about memes, jokes and gossips.

She had told her male bestfriend that she has a boyfriend, and never gossiped about him to her bestfriend. Though, she often calls her male bestfriend as 'hero'. This, along with them hanging out with each other, made her boyfriend extremely disheartened. She isn't cheating as she informs her boyfriend about her plans beforehand.

Recently, it was her male bestfriend's birthday where she surprisingly gifted him a watch along with a photo frame featuring both of them. 'Surprising' because she never took the same effort to gift her boyfriend anything near to similar to what she gifted her male bestfriend. At the most, she took her boyfriend mostly on dinners on his birthdays.

This made her boyfriend enraged with anger and they ended up in a huge quarrel which resulted in him being left on read by her.

Her words – "why can't I make him my friend? Who are you to decide about my friendships?"
She later called up her boyfriend multiple times to apologize but he did not pick up the phone. Her boyfriend is apparently way too depressed to answer any of her calls. It was his first long term relationship (about 2 years old).

Tl;Dr : My bf was asked for help by his friend and he is confused as to how should he fix their issues since it is affecting his friend alot?