What do you think about this scenario with my ex? serious answers only NO SARCASM please :-)?

I was with my ex for 2 1/2 yrs & we broke up in Nov. Our relationship was very serious & that's why i didnt want 2 burn the bridge just yet b/c we were best friends & we were each others first. We got along as friends for awhile after that & then he started messing with his co-worker a mth after we split. I have gone back & forth about if i can truly be his friend while he's in a relationship with this girl who proclaimed her feelings for him in Feb of last yr. About 2 wks ago he cheated on her with me. it wasnt planned & i felt horrible about it & so did he. He just joined a frat & the female attention will be a prob. When we were together he said he didnt know if he could trust himself not 2 cheat & now 4-5 mths after our split, he works with his girl (@ a new job) & is gonna move in with her. He says she supports him on another level. I just cut ties b/c it hurts 2 much. he told me @ first he's wit her 2 get over me. & now he acts like he cares a lot about her. what should i think??