S. -
I had a simlilar expereince happen. I became friends with my son's SLP. We immediately "clicked" and it was great to speak candidly about my fears about my son. She was so supportive. And, after about a year, she pulled away citing boundaries. (BTW - I've looked into the ethics issues and yes, it is a real thing.)
It was painful and I was hurt. However, the boundaries are there for a very good reason. She can't be objective about my son's treatment if we are friends, and I can't be objective about determining the right course for him. And, of course, there are times when we disagree and we need to be able to voice that, to work together, without a fear of jeopardizing my son's treatment. Now, we have a very friendly relationship where we can still empathize thorugh tough times, but don't go out, or have overly personal conversations. We know that someday she won't be treating my son and possibly then we can have a deeper friendship but for now, this happy medium has allowed us to both see things more objectively.
Now, with that said, I can also empathize with your feelings of loss about this relationship (on top of whatever loss or other feelings you may be having about your son). I encourage you to connect with other mothers that are going through similar challenges. This has reall helped me. I don't knwo what your son's issues are, but we have a parent organization and, if it at all fits with your child's profile, please join and come to some of our events. Check us out at www.chicagofloortimefamilies.com. If our group doesn't at all apply to your situation, you can still email if you want to talk. I'm sure we still have some things in common in terms of helping our kids to develop to their fullest capacity.
Also, are there other moms in your son's school with whom you can connect? It woudl be grat if you coudl organize a coffee clutch or a mom's night out where you can support each other throuhg the ups and downs of developmental delays.
It is so wonderful that your son has a teacher that you feel is good for him, and that you admire. that is a true blessing. And, clearly you've already established a strong communication. That is important. Allow for the boundaries, but know that your strong relationship will actually help you in advocating for your child, and help her to see your child as an individual. Be thankful.
Hang in there. It will be OK. Try to find the outlets you need. that is important.
R.