A married woman may have a love life or be in a long term committed relationship with her husband and can still be falling in love with another man. So, yes, a married woman with a love life can fall in love with someone else like her best friend for a short or long time.
What is it called when a married woman has a boyfriend?
Adulterer (or adulteress) can be used for anyone who has a sexual relationship with a person who is married to another, but again does not always imply the specific social situational that mistress does.
Can a happily married woman fall in love with someone else?
Can you fall in love with someone else while married? Yes, you can be married but constantly thinking about someone else or find new love or a new person. Its not that your spouse is a bad person its just that they are not the person you love anymore and you just dont feel in love with them.