Should i give him a second chance?

I recently saw my bf's inbox on fb where he wrote to one girl i love u tc mwah i miss u my baby.. n all that crap. i confronted him n he said that he was influenced by his mates to cheat on me. is that my fault? should i give him a second chance? we been together for a year now and realise that he means a lot to me.. wat should i do?