okay I need help/ advice about my friend. 

I have two best friends lets just go by the name of K and M. So K is in a relationship with her boyfriend since January. I like them as a couple but I always thought he is a like at out formal/ prom he made K cry because he left the formal to get alcohol using his fake id. I mean who leaves your girlfriend alone at your prom? But I didn't pay attention to it because he made her happy. Also M my other best friend is dating a guy who lives at the beach, which is 2 hours from where we live (we live in the city). M and her boyfriend have been dating for 2 years. To be honest I'm more closer to M's boyfriend then with K's boyfriend. We are the three boyfriends and we are always together. An me? I'm like the third wheel to their dates hahahah. 

So last week or 2 weeks ago, K and M went to a party. K decide to come with M because M just broke up with her boyfriend. So K and M went to the party and then the next morning I got a call from K saying she kissed her ex boy friend meaning she cheated on her boyfriend and her boyfriend knows and that she dosen't know the situation of them at the moment. 

So for a week K has been talking to The boy she kissed and her ex boyfriend. She made friends with another girl who use to be my best friend 2 years ago. We never fought but we became distant from each other when I found out how Nnoying she really is. Lets  go by the name of C, For a week C and K have become close. They are like obsess with each other. Leaving me and M alone. Anyways I don't know what happen but all of a sudden K has started lying to us, she would come to school late in the morning and tell M and I that she went to see her ex boyfriend. But then C would tell me that the night before they had a sleep over and they got up late and went to macdonalds for breakfast and then went to school. So she lied... and then there was another party on two days ago. I couldn't go so M invited K but K rejected it because she knew the boy she cheered her ex boyfriend on will be there. But guess what? as soon as C says she's going to the party, K says she going. Also what pissed me off is that when M and her boyfriend broke up, M didn't go to school. So I decided to skip class and told K to come with me because she has her drivers licence and we can stop at baskin and Robbins to get food for her before going to her house. But K complained to me that it was to risky and will get caught. but when C was getting for a formal for another school, K gave no hesitation to skip class and take Chloe home an come back to school? Like I don't get that? M our best friend was in need and she couldn't be there for her but when C could easily catch the bus to get her hair and make up done she took her across town to do that. 

there are more stories about K being like this but I just want to state that Idont care if she's best friends with Chloe . But what pisses me off is that she is lying to us, she's being a bad friend. Arghhhh some please tell me what M and I should do to our friend. I want to tell her off but At te same time I don't K to think it's because we are jealous or take it the wrong way. I don't want to lose my best friend because of this. I hate how C is changing K, telling her to Mendall with both boy's feelings. Arghhb please tell me what to do, should I ignore it and pretend its just a period because K has broken up with her boyfriend and she's not herself anymore. I don't know what to do!!

Like I don't know what to do... :( I don't get it M and I have been there for K since the day she and her boyfriend strted dating. When they broke up, M and I took K to frozen yoghurt in the mddle of the night... I really hate this feeling