Yes. you can halter train an adult cow. You can do it in 3 days.
The first thing to do is to get the halter on. If the cow is tame and enjoys being stroked/brushed/handled then it's pretty easy. Make sure you know how the halter goes on so you can do it pretty quickly before before the cow realises what you're doing.
If the cow's not particularly tame then you need to put her in a crush so you can get the halter on properly without a load of argument (on her part). If you don't have a crush or suitable way of restraining, then you need to spend some time with her until she is tame.
Once you've got the halter on, tie it to something really substantial, like a feed barrier, or gate post. Don't try leading her round the field at this stage and think you can hold her. Because you can't. And you will just teach her that she is stronger than you, and she won't forget. That is why you tie her to something immoveable, which teaches her that pulling against the halter serves no purpose and she can't win. Keep her tied up for a few hours to reinforce the point. Then let her go. Actually, at this stage I use a head collar and lead rope, as you can leave the headcollar on and easily attach and undo the lead rope as you want. But if you only have a halter then leave it on and secure the end, unless your cow is particularly tame and will easily let you put it on again.
Tie the cow up again for a few hours the next day and again the following day. By then you should be able to undo the rope, but keep it taut and pull in the direction you want to go, and the cow should follow. It is important to keep the rope tight in the early stages, as if you let it go slack the cow can get its head away from you and will build up some momentum before you can stop her, and you will lose her. Do not try and stop a cow that is running away from you. You may end up with rope burns, and you will certainly fail, and will have taught the cow that she can escape from you. Better to let her run and make out that you allowed her to. But when you get the chance, pick up the rope and quickly tie it again to something imoveable for a while, and have another go. This time, keep the rope tight!
It is customary to lead from the animal's left. So initially pull her round to the left, ie anticlockwise and that way you will keep contol of her head and she will learn that she has no option but to follow you.