This heifer has learned that by bolting she can get away. And so, when she has had enough, she bolts and gets away and therefore is rewarded for bolting. I am not sure a donkey will help this situation as the heifer is not afraid of you, she just knows she can get away. Once she learns she can not get away from the donkey, she will not try, but she will definitely try it once you take the halter. Bovines are far smarter than most give them credit for.
I hope I can describe this method as it helped me greatly back 30 years ago when I was showing Limi's. Put two halters on her; one with a long lead (12 feet) and the other a normal lead. Have someone QUITELY wrap the long lead around a post about 8 feet away. Then untie the heifer and lead her. If you make the 8 feet. Tie her up. Brush her while the other person take the rope another 8 feet away. If the heifer bolts, try to hold her long enough for her to hit the end of the rope. If she bolts hard, be careful as she may flip and take you down with her. As soon as she has hit the end of the rope and is on all fours, quickly BUT QUIETLY, get up in leading position again and act as if nothing happened. Tie her up, brush her as the other person advances 8 feet and reties. Your goal is to get her to think that YOU stopped her. This entire process is to be done very quietly and calmly. I am not joking when I say it usually takes about 2 sessions of about an hour of this to completely rid the bad bolting. A caution: always be aware of where the other rope is. Do not get tangled up! I have found it best to keep the long rope on the left side (leading side) and in front...
If you have another person who is experienced and is quick, you can have them advance 8 feet without tying off until she bolts.
Do not get injured!! This is a proven method but takes at least one knowledgeable and experienced person. Good luck.