My best friends ex asked me out .. Help ?

Ok so last night i was a bit upset about some issues and then my best friends ex heard about them and asked was i ok and he really cheered me up. We we're talking for the whole night and had a wicked good laugh. He went out with my best friend on and off for a year and they've been over with a half a year now . My best friend is also happily with another fella with a couple months now . Even since her & the ex broke up ive stayed friends with both. After the great convo's we've been having in the past month i started to like her ex. I know he likes me .. he makes it very obvious :L but he's so nice too. I dont know what to do :( I love my best friend but would she get mad? This ex was really important to her and now she hates him . He didnt cheat on her or anything i thought he treated her really nice but they just got bord of eachother .