The main purpose of a walking (zimmer) frame is to provide support to the user, it can help with keeping balance, it provides general support so that the person can move around as well as using it outside and it is useful also for when the user wishes to stop to take a rest as it provides something for him or her to hold onto.
Assisting someone who is using a walking frame is limited to being nearby for the user, you can help by carrying anything for the person that they might attempt to carry themselves while using a frame and while indoors it will definately help to remove any obstacles from their path.
I feel that trying to assist a walking frame user in any other way than those mentioned above might actually be less helpful, for example if you try to move the walking frame along with them you could cause them to feel less stable and less in control, it could also be dangerous.
A user of a walking frame needs time to get used to walking with it, knowing how much they need to move it and finding a way of using the frame that they are comfortable with is something that might take longer if you try to assist too much.
Even when it comes to mobility and wanting to do everything that you can to help someone, its sometimes better (more helpful) to step aside and to let the user do what they need to do.