Relationship help? a liar orr whaa?

so all was goign really dandy with my bf until one of hes friend[ not a close one] told me that the only reason why he went out with me was to make this other guy jealous and to show him that i had feelings for him. he also told me that he was going to talk to my sister when she came back and shii. however im a bit skeptical to believe him because i dont think a guy would talk to me for two whole months just to make someone else jealous and wouldve bought me a bear for valentines and stuff. should i believe hes friend or what?