Is it healthy or unhealthy to be friends with your older kids when you are a single parent?

It sounds like you are fortunate to have a wonderful relationship with your daughters. As a single parent myself, I believe that there is no “right” answer to this question. Saying that though, I do believe that it is very important that our first and most important role is to parent our children. This means providing guidance, setting boundaries (and enforcing them), and making sure that our children have the competencies and values necessary to be productive and functioning adults. Our kids also need to know that we are a safe space and that they can turn to us if they are struggling or if they find themselves in trouble.

Getting along well, spending time together, and being able to talk about all sorts of things does not only fall within the friendship realm. A good parent-child relationship can provide these same benefits and we can conclude that we are not only parents to, but also friends with our child.

Being our kids’ “buddy” may sound like a great idea, but it can also backfire. If we side-line our parenting because we are worried that it will alienate us from our kids, I think that we do them a disservice. In my experience love, consistent discipline, and a nurturing home environment are the keys to ensuring that we grow to have good and healthy relationships with our adult kids in the future.

You are so lucky that you enjoy such amazing bonding with your daughters.
I think there is no single right way of parenting. Our parenting philosophy usually stems from our own experiences as children and what we observe around us.
Research findings indicate that gentle parenting is the most effective style. Gentle parents are very responsive to emotional needs of their children, enjoy strong bonding with them and spend quality time with them. At the same time, they have established rules and realistic expectations from their children.
I think you have been a very mindful and gentle parent. You have made your girls confident, loving and responsible.

What a special relationship you have with your girls. I firmly believe that relationships are so important but that a parent is so much more than a friend. Whilst it is wonderful to have the kind of relationship where you enjoy spending time together, there will also be times when you are required to be firm and apply expectations to keep your child safe and well. I see no issue with having a great time together, as long as you are not behaving inappropriately, such as drinking alcohol with your child. There must always be some boundaries that can't be crossed and a parents role is to prepare their child to function on their own in the world. This can be done through building a strong relationship with your child but also being clear about expectations.

I don't know that there is evidence about maintaining a safe distance in being friends with your kids except that the concerns raised in this space are more about the damage that can come to children when the adults rely on their children to fill the emotional hole in the parent that has been left by the other partner.
In your case, it doesn't sound like that at all and in fact, it sounds like you maintained your adult-parent boundaries while allowing a friendship relationship to thrive with your girls. It is a tricky balance to pull off but the relationship you have with them now, as adults, is telling enough that you have done this so well.