How best to handle a summons regarding a credit card debt?

Your mother-in-law may be, from what you write, effectively judgment proof: that is, her only income is exempt is garnishment, and from you write, she was few assets which a creditor could reach. If they had not sued her yet, you could do what you suggest; but because they have sued her, a more formal response is required.

Your mother in law must answer the summons. She can answer it herself, pro se, or she can hire an attorney to answer it for her. You may not answer it on her behalf, since you are a non-lawyer--for  you to answer a summons for her would be for you to practice law without a license, which is not allowed. While she could simply ignore the summons, let them get a default judgment against her, and then rely on them not being able to collect, a better course is to submit an answer which either denies outright anything which can be denied, admits only the harmless basics (like her name), and neither confirms nor denies the other elements of the complaint.  That will keep the case rolling forward, which will provide an opportunity to speak with opposing counsel and try to work something out; and indeed, given how long cases can sometimes take to come to fruitition, it's not impossible that your mother-in-law may pass away at some  point before trial, effectively mooting the issue (the creditor could still try to sue her estate, but at that point, there is no impact on her personally).