When you invite someone to a restaurant for their birthday and you are explicitly offering to pay for them, it is generally understood that the invitation is extended only to the person being celebrated (in this case, your friend) and not to their spouse or any other guests. Therefore, you are not obligated to pay for her husband or any additional guests who were not part of the original invitation.
If your friend's husband assumed he would be included in the meal, it might be a misunderstanding on his part. In such a situation, you can politely clarify the situation by explaining that you intended the invitation to be for your friend only. If you are comfortable with it, you can also let him know that he is welcome to join but he will need to cover his own expenses.
Communication is key in such situations to avoid any potential misunderstandings or awkwardness. Kindly and respectfully explaining the situation will help everyone involved understand the arrangements for the birthday celebration.