Bro,
If she has a bf, you had better stay away. The ROI in not worth
it.
If she has a bf who is two-timing her, and their relationship is
suffering, you had better bloody stay away from this mess.
The worst time to tackle a girl is when she is in a mental mess. At
best, she will view you as a good friend. Once you have become her
friend, you will not have any chance of making her your gf. The
moment she sorts out her problem with her boy-boy, you will cry
till your tear ducts go dry.
In a worst-case scenario, you become her rebound guy. Like a
spring, you help lift her from the abyss. However, once she has
fully rebound, you are no longer needed. Negative returns on
investment, my friend.