L.,
I lived in Texas for almost 10 years before moving to Arkansas and homeschooling is popular there. It has also fascinated me because I am a former school teacher.
You are not going to want to hear this, but the situation you described is very common. The majority of home schooled kids I talked to in Texas (and I talked to quite a few) gave similar replies.
There are guidelines and recommendations that the state gives to PUBLIC schools to teach, there are tons of homeschooling curriculums someone can purchase if they want to, but your stepdaughter's mother doesn't HAVE to do any of it. She is not held accountable for her teaching in any way by the state and she can take your step daughter all the way up to 12 grade without having her take a standardized test if she wants. I have seen it done.
Many homeschooling moms just have their children take a GED at 18, some don't even do that much. I am not anti-homeschooling, I just think a lot parents don't know how to prepare their children academically for college and the workforce of today's society and they do more harm than good by homeschooling. (Not all, I have seen a few great situations, but they are very few.)
If your husband is worried, I would suggest he ask his ex what curriculum she uses and encourage her to purchase one if she says she just makes up her own. (Yes, a lot of people do this and forget entire subjects and skip grade levels. The internet can offer more curriculums than you could imagine if you google "homeschooling curriculum".) Encourage her to join a homeschooling group where moms support each other and give each other ideas. They are easy to find locally. www.homeschooling.com is also a good resourse.
Find out what ISD (Independent School District) your stepdaughter's home is located in and look up their website or contact them directly to get the list of grade level standards and requirements. This will help you better see where she should be academically.
Best wishes,
S.