Clarify whether money will go into the company, to the selling owner or to both. Raising capital for operations and selling an existing equity interest serve different purposes.
An ownership percentage does not explain the whole governance arrangement. Decision rights, responsibilities, information and future exit mechanisms also matter. Remaining a shareholder and remaining a manager are separate choices. Evaluate the part of the business you retain, the relationship you will have with the new investor and how disagreements or a later exit would be addressed.