1. She is so into you that everytime she comes across you, which only seldom happens after you left that hospital, she develops a tunnel vision which includes only you. Apart from being her sole focus visually, she did her best to build a connection with you verbally without any distractions.
2. Even if she hadn't talked about you (including a study of your facebook page
) with her friend up to your encounter (noticing then that her friend liked you), which is highly unlikely, she did notice her delightful response after seeing you in person for the first time.
3. Her friend is in a much better shape and she is working hard to lose her excessive weight, so her insecurity, jealousy and contempt for other females is skyrocketing at this stage of her life.
4. She has got the hint that she is not wanted by you (long before today) AND she also realized that you were liking what you saw as far as her friend is concerned.
The fact that your friend didn't introduce her friend in the first place when she should, as the natural progression of things would be after you had introduced your friend, reminds me of the scene from the Matrix: Neo saw the same black cat twice, a deja vu moment, which was a glitch in the Matrix. Likewise, when you noticed the absence of the introduction, you understood that something didn't go well with your friend. She had already noticed the connection that was building between you and her friend from the first time you talked to each other and her decision not to introduce her was just encouraged more and more during the intervening period until your second and last small-talk.
I speculate that one of them, most probably the one you already knew, inquired about your appearance in that gym (if you have left your gym to join this one etc.) so that she would have a way to meet you rather often.
Also it is very probable that they extended their training time to give you the opportunity (the SPACE) to say farewell to them. In addition to this, they strategically placed themselves (doing squats) near the water fountain, as it was almost certain you would pass by before leaving. Thus, they ensured you wouldn't "forget" to say goodbye.