The more details I hear about this, the more I'm swinging toward inconsiderate (I previously posted "somewhat inconsiderate.") My reasons are:
1) I originally thought that the husband had been unavailable to the family for a while (extended business trip, working 80 hour weeks, etc) and this was a last-minute opportunity to seize some family time, so she took it. But her DH had been available to the family but the family had prioritized a household project above spending time together as a family. (My DH is working on finishing our basement and is spending nearly all of his free time down there. I know how it is, but... )
2) The OP knew that her DH would be available on Monday night, but didn't immediately decide to take the opportunity to spend time together. It was only when they saw the beautiful weather that she decided to cancel her previous plans. If they were that starved for family time, I think they would have done something together regardless of the weather. This is definitely sounding more in the "better offer" category, rather than an "unexpected opportunity that I just couldn't pass up."
3) In my opinion, canceling the morning of an event is "short notice" and should be avoided. Sometimes it's unavoidable (emergencies, etc), but in this case I think it was avoidable and was inconsiderate to the host who was planning on your attendance.
I don't think it's worth ruining a friendship over, and I wouldn't be "furious." I am glad that the OP's friend has calmed down. But since the OP asked if we thought she was inconsiderate, and I think she was. I am not sure that I wouldn't have done the same thing, but I would recognize that I was inconsiderate to my friend and wouldn't be surprised if she was upset.
I get frustrated by people who say things like "my family comes first" or "my marriage is my priority" as a way to justify their rude behavior. My family/marriage is very high on my priority list, too, but to me that would mean either declining the plans with the homeschool group in the first place (or at least calling *as soon as* I knew I had an opportunity to spend time with my husband, regardless of weather), adjusting the schedule of the home project to allow for "family time", or working together on the project so you still have "family time."