I don't know why it is, but we humans have a tendency to want to rain on other's parades. We all have a bit of a jealous streak, some act upon it more than others. Even the best of friends can be pretty catty. The closer the friend, the more they know where the soft target is.
Whether by choice or chance, your friend has no children. If it is by choice, perhaps she looks at you and has twinges of regret for not having children. If by chance, as you were, you may be all too familiar with a feeling of longing for one of your own. In either case, perhaps she wants to show you that she could be as good, or better mother.
Also, if she's in social work, she sees much more of the negative side of parenting on a daily basis. It is only natural that she tends to see the negative side and zero on it. It's a quite publicized fact that many social workers have high burnout rate and often need counselling themselves after the years of dealing with troubled families and individuals.
Your friend seems to be offering unsolicited advice. If it's the envy thing, she has the added ammunition of being able to say she is trained in these matters. What your friend needs to learn is the same thing a lot of friends, sisters, moms, mom-in-law, aunts and the neighbor need to learn..... Only give advice when I ask for it, or when you see I am truly in over my head. Alas, we are women, we are moms, we must endure the comments of the other women who love us. We must realize no matter how much they love us, they have that little streak and must show us how they could do it better.
If your friend is childless by choice, you might reply, if only to yourself, "I'm making notes and going to save them for you when you have kids". If by chance, then it is good to be compassionate and not ever say anything to point out she doesn't have kids.
Whatever the case, when the unsolicited comments come, you could simply say, "Thank you my helicopter friend for hovering over me."
We all live with the mostly unspoken fear of being one of those moms in the ER saying over and over "I only took my eyes off him for a minute". Your son is 17 months old. It is absolutely necessary you are aware of his whereabouts at all times. My youngest discovered at age 2 how to unlock a deadbolted door, get outside, strip naked and sit in the road. I got there just before the concrete truck that was barrelling toward her. I had gone to the bathroom, came out and thought she was in her room playing. Just happened to look out a window and see her. I had only taken my eyes off her for a minute. I have a friend who went inside from outdoor play in a fenced, childsafe yard to bring out a drink. In two minutes or less, her daughter had stuffed her nose full of some pebbles she found in the sandbox.
Child safety products exist, child safety warnings are mandated due to tragedies. Children are curious and don't realize the dangers that everyday objects can pose. We cover the outlets, put latches on the cabinets, raise the drape and blind cords out of reach, put the plastic bags in something sealed. That doesn't even begin to cover their safety issues. Doors closed on fingers, trip and fall against the coffee table, thinking someone's left behind dog poop in the park is candy, on and on. Those things are not helicoptering, they are a necessity. It takes true vigilance to stop what you are doing a hundred times a day, just to take a look and see they are ok.
As for the other comments about your parenting choices, just the thought to yourself, or verbal reply to your helicopter friend may be of help to you. We are individual families with individual ideas and goals for our families. We all just do the best we can with what we have.
So, when your friend seems to be coming off as critical of your parenting, just keep in mind she may be professionally trained, but that in no way equates or prepares you for the real day to day. While your friend may have ideas of what her ideals and goals would be if she had a family, this is your family. Thousands of books, courses, observing others, family traditions. We sort through these and use the things that seem to fit what our goals are for our family. If you are content with how things are working in your house, nuff said. Don't judge yourself and don't let what others say cause you to judge yourself. We moms tend to second guess ourselves way too much as it is. If what you are doing isn't working for you, you'll figure it out. If you need help, you'll ask for it.
One thing to always remember, women are women. Even the best have this one flaw in common. Your friend may love you dearly BUT.... If there's a flaw in you, we'll find it. If we can't see one, we'll invent one for you. We range from those who just have the thought, let it pass, to the mildy catty to the very catty to the downright mean about it.
I know that I loved spending time just watching my kids. I was fascinated. There is the watching them for safety's sake, of course, but there is also the joy of seeing them discover, learn and grow. If you're not watching, you may miss something fabulous. So many things they will only do once. And everything, they will only do the first time, once. How lucky you are to get to be there to see it. Too soon, they go to school and someone else teaches them, sees the "firsts". Their world expands so quickly and our place in it becomes less and less. Revel in this time, enjoy it while you can.