The fact that she owns 45% likely means that you will have to agree on the value of the good will of the business, if any, and the value of the assets of the business. Assuming you can agree then you would buy her out for a price upon which you agree. Not necessarily 45% of the agreed total value business. It could be more or less. If she states she doesn't want to end her involvement you would have to file a lawsuit to dissolve the business.
This could be an expensive proposition as the business value would have to be determined. Each side might hire experts to value the business. There are lots of ways to value the business.
Buy–sell agreements consist of several legally binding clauses in a business partnership or operating agreement or as separate agreement; it controls the following business decisions: Who can buy a departing partner’s or shareholder’s share of the business (this may include outsiders or be limited to other partners/shareholders); What events will trigger a buyout (the most common events that trigger a buyout are an owner’s death, disability, or retirement. Less common and more problematic events include a desire of one owner to exit the company or an owner getting a divorce) and; What price will be paid for a partner’s or shareholder’s interest in the partnership.
A buy-sell agreement would have been prudent. Check and see if you guys executed one. It can be in the form of a cross-purchase plan or a repurchase (entity or stock-redemption) plan since the business is a corporation.
You probably don't have one as not having a shareholder agreement which would dictate the purchase price of a departing shareholder is very common with small businesses. People often form a corporation with an online automated company or the like or by using the services of a company they found in a newspaper ad. This can be a disaster.
Without a plan any shareholder who owns at least a third of the stock in a small corporation (and at 45% she does) could force an involuntary dissolution pursuant to the California Corporations Code. Under such a proceeding, the remaining shareholders (in this case you) would be given the option to buy out the departing owner at “fair value.” “Fair Value” is subjective and has been the source of a lot of legal fees in lots of lawsuits. It is not “fair market value” which is more easily determined, but it is a starting point for negotiation.
If your company is making good money, then you should separate from her now. And you will likely need to employ a lawyer. If you're not making a lot it might not make sense to spend funds on a dispute. The particular facts of your situation should be discussed with an experienced attorney.