Hi angel fish :o)
I had the mirena coil fitted back in january. You should at least once a month check yourself to see if you can still feel the string. This is just to see if it has moved. The phamplet said that if it can be felt during intercourse then the strings may need to be adjusted by your Doctor. I went to the Doctors only last week to have mine checked as I was worried it had moved, it hadn't think I was just being a bit over cautious.
Your friend may just need her's adjusting.