**checks LinkedIn** nearly 3 years. If it happened on a call I typically say something along the lines of, oh year, I know them a little bit. It looks like they have some good folks over there.
If they are not working with them I might ask them something like, they are an interesting brand, what stands out to you about them. Or if they are working together, ask them what they are doing together.
Over email would be a similar situation. Not reacting negatively or badmouthing, if anything complimentary of the other team and inquire what they are doing. If you can find an edge of like "yeah they do a lot of X, but I didn't know they did anything with Y" and get the prospect to offer up areas where you can play, that helps.