While I don't agree with the parent, accusing her of being "in bed" with another person or group is not sexist – it's a common idiom which refers to having an undesirably close or underhanded association with others.
Nearly every time I've heard the phrase "in bed with" it's been in reference to men, and the meaning is hot man-on-man collusion (not sex).
However it is a really unfortunate phrase to use where the group is mixed gender. Especially so when it's an imbalanced mix, such as very few women.
(I think everyone should get a small stack of "Yikes did I really say that?" chips. Only after someone runs out of their chips should we bring out the pitchforks and enjoy ourselves a nice cathartic public shaming spree.)