Answer: My believe concerning a person's sexual orientation is that their orientation is dependent on what their needs are at the time, and who is available to fulfill them. Your needs may be different now then when you were younger, and you may be more comfortable with your sexuality.
Fantasizing about sex with another woman does not make you bi-sexual or lesbian in of itself. Being attracted to a woman you meet in public increases the likelihood. It is hard to deny a physical response. Finding a woman physically attractive is not the same as finding her sexually attractive. Looking at and finding a girl attractive is not the same as becoming sexually aroused when you look at her.
Was your masturbation experience with your friend awkward because it was the first time you did it with a woman, or the very first time with anyone? Since you enjoyed it, why not try it again.
As long as you are honest with your friend about your feelings, you should not have to worry about the end result. Make it clear you are curious but unsure. Take the time to get to know her in person, go out on a date. Talk and make sure she is one of those one in a thousand women. If there is no spark, do not have sex with her. As when you were a teen, start with holding hands and a kiss. Tell her you want to be in control. Then explore body massage, then sexual touching, and then perhaps oral sex. Go slow and do not have any false expectations of yourself or the experience. Make sure she does not go too fast for you.
Be sure that you really want this and are ready for the reality that you may enjoy it and perhaps even prefer it to sex with a man. Some women try it but deep down hope they will not enjoy it and the experience turns out badly.
While her being a woman increases the chances she will know how to make love to you, be aware that lesbians are not automatically great lovers. Just because things do not click with her does not mean you will not enjoy sex with another woman. Do not base your entire sexual identity on this one experience.