The best way to maintain a friendship is not to involve money in it.
1. Discuss all obligations and areas of responsibility in advance. So that it does not happen that one of you works more, and the other thinks that it is she who invests more in the common cause, you need to fix all the rules on paper in advance. Write down who is responsible for what, who has what responsibilities and how profits will be distributed. It is also worth discussing vacations, sick days, maternity leave in advance - even if it seems that it is easier to solve these problems as they come.
Don't rely on your own memory. In a conflict situation (and they will certainly be) it will be easier to refer to the written agreements than to argue over who remembers what and how. Also discuss in advance who gets what if they decide to go out of business. This will make your life much easier if this situation really happens.
But with a friend who became your business partner, everything is different. You may find that you just don't have any topics for conversation outside of work.
3. Set aside time that you devote to friendship, not work. Agree that, for example, after 8 pm you do not discuss business matters, or that you are going to a friendly dinner on Sundays - and not a word about business. If you do not distinguish between work and personal time, then the first will gradually absorb the second. In order not to become workaholics and have time to communicate outside of business, you also need to talk in advance.
You will be interested to know how to keep love in a couple if you have a joint business.