I have followed this thread with interest and thought long and hard about how to help you. I have been on both sides of the story. First let me say in no way do I condone your friends behaviour towards you. Sadly, she has misplaced her anger at her situation on you and it is not your responsibility.
Due to ongoing medical difficulties from when I was 14, when we were married we were told not to wait too long before trying for a family. We started trying after a year. Nothing happened so with the care of my GP we were sent for further testing. sadly, things were not good. After a year of tests we were told we could try IVF but had less than a 5% chance of concieving through that and really should consider adoption. This was devastating. Through all this the only people who knew what was happening were our parents. Because at the same time our sisters, brothers and close friends were all starting their families. We have a SIL who was on IVf and broke down whenever she heard of a pregnancy. We saw how that made the excited parents to be feel so we decided not to take away anyone's joy by telling them that most likely we couldn't have kids. It was hard sometimes but I still maintain it was the right thing to do. (My SIL became nastiewr and nastier towards people expecting). We decided to try IVF anyway and after a couple of years had a success against the odds!! By this stage my SIL knew what we were doing as we went to the same clinic. She had already had a success and when I concieved her son was 2 years old. Nevertheless she kept ringing me telling me she resented me for being pregnant and it should be her baby. (It got worse but that is for another time). One of the best things about IVF is that it comes with counsellours. If your friend has started IVF I would urge her to speak to them about how to cope with other people's pregnancies. DH and I would always have a private cry at home when he heard our friends news but it was never about the friends, just what we couldn't have and wanted so badly. I used to feel guilty even for the private cry so one way i got over that was always to write our friends a letter telling them of our joy at their news and how much we looked forward to being involved in their baby's life. It really helped me without the parents-to-be knowing our pain.
Even though the odds were against us we decided to try for a year for one more bub. Again, parents, best friend and this time my mothers group knew what was happening. It made a big difference having the extra support. I told my mothers group why we hadn't told in the past and that I didn't want them to hide any news they might have from me. At the same time a good friend is on IVF after several miscarriages and after 4 years no luck. I was scared to tell her that we had a second success. When I told her at 12 weeks she was overjoyed. She said I give her hope. (Again, same clinic so she knows what we have been through). Yet my SIL has been nastier than ever!
You do not have any obligation to tell your friend you are trying for a baby. That is no ones business but you and your partner. I would urge your friend to seek counselling, either through IVF or other. You are not going to be the only person she meets who is pregnant. She needs to learn coping mechanisms. Also, a good friend does not put conditions on a friendship - like you should only have two babies. That is not a friend.
Sorry for the long post. Lastly, I wish you much success TTC#3 and look forward to reading a good news post when you do.