If you have your own business, do you have any experience that your friend asks you to help for free in your field of expertise? They would expect you to help them because you are friends. In such a case, you might help them, and I think it is no problem if it is once or twice. Or, if you really want to help them without any reward, it will be great. Otherwise, you should charge them even if they are your friends.
To conduct business is to make money, which is not volunteering. If you want to do volunteer, I have nothing to say. However, if you want to do business, you should keep in mind that you are a business owner. Assuming that you continue to provide your products and services to customers always for free, you cannot live, and you will have to stop providing. Then, your customer might get upset if they really want your products or services.
In a more specific example, if TOYOTA gives their cars always for free, it will go bankrupt soon. When something like this happens, not only customers but also their employees will be in trouble.
Therefore, it is not always good to provide your services with friends for free. You should charge them an appropriate fee for your services. Moreover, a true friend wouldn’t always ask you to help one-sidedly without any reward. And it might be better to rethink the relationship with them.