You haven't sat down to really talk with both people (the husband and the friend) to assess if a healthy, ethical polyship is "doable" here. It was just an initial temperature check. Glad that part went well.
WHY are we thinking of doing this? Is everyone wanting it because they want it for themselves Or because YOU want it and they want to go with what YOU want? Could sort that next.
After that? Could take the deeper assessment to map out the WHAT and the HOW of it. Could read things together, sort it out. If it's a good investment risk, go on to trying it on. If it's is just too incompatible, stay friends -- friends grown closer for having thought this out together.
What open model
are you thinking of? Will you be the only one with 2 partners or will they also date at some point? Can all of you thrive in the open relationship model you are considering together? (versus merely survive in it?)
How do you each cope with emotional management things like jealousy?
Or more jealousy?
are they prepared for? What about polyhell?
If it goes wahoonie, how do you each want to be as exes? Everyone ends up single? Still friends or part ways and never speak? If you can't talk about that now when things are good, you don't want to first talk about it when it is already happening.
Look before you leap.