Yes, we did. My kids were in Catholic school through 2nd and 4th grades. That school used one of those spiral programs that all the institutional schools seem to use. When we began home schooling, I gave the kids the placement tests available on the www.singaporemath.com website. The transition was hard, but well worth it.
My older placed a level lower than his "grade level," so that's where he started. I used the Challenging Word Problems as a daily supplement (1 page a day, from the previous level's topics). He did great with that, but then he's the kid who wants to be a mathematician. He's currently doing high school geometry in "7th grade."
Now, my younger was one of those "good at math" kids who didn't really understand what she was doing -- just skating along on natural ability and some luck. She tested into her grade level in Singapore, but she struggled for a few months with it. We took it really slow and did every problem step-by-step for a long time. Word problems were the most frustrating part since solving those is a whole separate discipline of itself. Taking it really slowly and making sure she got every step before moving on was the key. Now she is truly good at math, and though she will never enjoy it as she enjoys making up stories, she can do it and do it well.
For your friend's 6th-grader, it might be more complicated. There's bound to be a lot more buildup of self-esteem/hormones/etc.
I would suggest that your friend consider using the CWP books a couple of levels back as their own supplement without changing the entire curriculum. She could even do a special unit just on solving word problems to kick it off. Regardless, she should expect to spend a lot of time sitting by her students as they learn word problem skills -- as I said before, that's an entirely separate mental discipline from knowing how to handle fractions or whatever. But all real-life math problems ARE word problems, so it's an essential skill IMO.
Karen