I saw in one of your comments you mentioned that your friend also has a boyfriend.
Essentially you have two people cheating on their significant others. I suspect part of the appeal for your friend is he is her boss as well as being married. (Forbidden fruit always tastes the sweetest.)
Secrecy & obstacles have a way of intensifying romance.
"No one understands us...We never meant for this to happen.. We met the love of our lives (at the wrong time)...Life has played a cruel joke on us..." All of sudden the cheaters want sympathy/empathy!
All the above are standard clichés cheating lovers use to define their circumstances. The reality is cheating is a choice!
It didn't just "happen". They (wanted) it!
Your friend doesn't care about his wife and I'm sure he doesn't care about her boyfriend. In fact it creates a "perfect cover" for them.
Contrary to popular opinion statistics show that the majority of cheating relationships take place in the workplace as oppose to online connections.
When it comes to dating/relationships most of them end in breakups. In fact it's been reported in the U.S. that divorce hovers around 50%.
If there's a 50% failure rate after exchanging vows with someone you can only imagine how the failure rate is among people having flings.
Nevertheless I have no intention to be judgmental.
All you can do is find out if your friend is being delusional which appears to be the case if she believes she is "in love".
The only reason most married men would leave their wife for their mistress is because they were (forced) to! The wife wants out!
Generally speaking most cheaters don't cheat for the purpose of (replacing) one relationship with another.
Their goal is to hold onto all that is "good" in their primary relationship while addressing their other "needs" whatever they are on the side.
Essentially the "other" person helps them to STAY or tolerate a relationship that is missing some elements they deem important.
Heartache ensues when one believes they are going to step into someone's life become the stepmother/stepfather and live happily ever after.
For the average married couple a divorce represents a major change in their living standards. This is one of the reasons why some people choose to cheat over running down to the courthouse to file for divorce, moving out of their home into an apartment, pay child support/alimony, become a weekend dad, and divide up friends/family as they choose sides.
Cheaters don't expect to get caught!