Adding authorized users to credit card account to boost positive credit history?

I am preparing to purchase my first home. That first home is not eligible for an FHA loan because it is too expensive. We're so close to be able to get it, but my credit history is limited to only one credit card, and car loan and they want more positive history. The loan folks suggested I get added to an credit account with great payment history (no lates). My mother in law gladly volunteered to add me as a user. She has perfect payment history on her credit card of 3 years. Her husband is going through a bankruptcy for debts he accumulated prior to their marriage. She doesn't know her credit score either. Does her credit score even matter if we are added onto this credit card account? She thinks she has over 700 but she's unsure as she hasn't checked in a year or so. If her score is low, but her credit account is perfect, will it affect us negatively or positively?

Read more: Adding authorized users to credit card account to boost positive credit history?