If you really love your friend, keep it in the friend zone. Why would you want to complicate her life and interrupt her good relationship with her partner? That seems you love yourself most and is a selfish endeavor.
No. If you really love her, you'd respect her happiness with the partner she already is with. Remain friends, but treat her and her partner with respect. Everyone is saying yes, tell her, but if you had asked, I'm in love with a married man, should I tell him, you would get resounding answers of No. Why should it be different in this circumstance!?