There are several cases I know when a best friend gets sweet to the boyfriend such that the woman suspects something. My advice is to be discerning because sometimes when fits of jealousy hits us, we lose our sense of judgment. In other words, we see things differently when we feel jealous. But when their behavior is obviously sweet to each other, that is a different case. There is a saying that a flirting woman should not be blamed but the man for his opportunistic attitude. If the best friend flirts and the guy bites then it takes two to tango - they are both at fault but the man is more to be blamed.