I think you need to do what you need to do.
Some kiddos are rougher, but there are other parts of their personalities which win our kids over, and let them choose to accept the bumbs and bruises. At this age,there is no great way of saying "I know you like so-and-so, but they aren't gentle with you, so we can't play any more." If we make a decision to curtail playdates with a certain child, we really can't put it back on anyone else, including our kids. And because we sometimes really like the parent, it does us well to keep our own counsel on the matter.
If it were me, I'd just give a good and honest reason why you might not like to have a playdate and leave it at that. This is something you can feel solid with. And then invite this mom out for a coffee or a drink. Chances are, she's dying to get out too and might be happy to just connect in that arena, without the kids.
I do find that the novelty of playing with another child-- even a child who hurts them-- has a big impression on children, and they will regularly ask to play with kids who would not be our first choice as playmates. But we are their parents, and there's something else we may not see that's meeting our child's needs, so it's good to be critical and try to be impartial, almost clinical, sometimes. Just to separate our preferences from our child's own. One thing I try to do is see the big picture, ask myself "how many times has my little guy been a pain in the neck to the other kid?" It give me some real perspective.
I'd say, if you want to further your relationship with this friend, do things on your own for now, or come to peace with 'what will happen, will happen'. This philosophy is something I practice a little more easily with my preschoolers than my son of the same age (because I'm his M. and so beloved of him, I just think my kid is the bees knees, so yep, I'm biased). So I know it's difficult. Choose what feels more comfortable for you, and then move forward in your friendship. The rest will likely fall into place if you do what feels right.
H.