Why would the barn owner cut your horses mane? I'm just wondering...
Anyway, you could cut it shorter (the longer parts) then pull it the rest of the way... I can't really help that much... I cut or pull my mares mane every November/December and let it grow back out through summer... so I don't know what to do when it's all cut at different lengths.
My cousin once cut one of our horses manes and it was so bad that we had to roach it, but it looked ok roached... that's always an opton, but if you want your horse to have a short mane, then pulling it would probably be best.