my best friend last year said she was over a guy and well i went out with him. i lost my best freind for the time i spent with that boy. anyway when i broke up with that boy me and my best freind became friends again and are closer than ever. i kno this didnt really help but if i could go back i regret losing my best friends for the time with that boy/.

If you don't think she's over him... don't do it.

Or really talk to her about it.
If she tries to get mad at you about it just tell her that you had the decency to actually have a serious talk with her about it instead just going out with him and hurting her.
If she's a good friend she'll understand that you are trying to be respectful of her feelings.

Also; think about your friend's relationship with that boy. Why didn't it work out? If it's the guys fault, you should make sure he won't hurt you like he hurt her.

There's a lot of factors to consider. If you need anything else. Just ask.

i think it all depends;;
ask her first,
make sure its been more than a month