I am kinda laughing at "strange toys" that there is no way for two kids to play with. I think I need an example.
But anyway, I would bring something from home, playdoh like some suggested or a ball or bubbles etc. Maybe something that the 6 year old could *teach* the 3yo. If your dd can't blow bubbles yet, bring bubbles and say "6yo can you show 3yo how to blow bubbles? Can you teach her how to do it? Ok let 3yo try. Wow you are a really good teacher 6yo!"
Or bring 2 coloring books from the $1 store and some crayons, "hey I got you guys these books to color here's one for you and one for you have fun!" etc
You could tell your friend and her daugher "we are working on taking turns/sharing with 3yo. Can you 6yo show 3yo how you take turns and share? She looks up to you because you are older, if you would help by taking turns sharing she could learn a lot."
6yo are kinda bossy. So I think that's normal, but maybe if you set them up with something so that she can feel *important* she might be okay.
Also when it happens that 6yo doesn't want to share, if saying "can 3yo have a turn when you are finished?" doesn't work, turn to the other mom and say, "yeah sometimes 3yo doesn't want to share/take turns and we are working on ways to deal with that. What do you do with 6yo when that happens?"