The friendship hit is a big concern that should definitely be examined.
Are you close to each other? Would you be hurt if the relationship fell apart? If you and she are practically sisters than is the risk of that ending worth the benefit to your relationship?
Sex is never just sex in my opinion. But if there isn't any long term plans for the sexual relationship to go on, then it might manage.
Another thing I notice is that you don't think your husband has the mental stamina to do this. I would personally talk with him. If he seems like he's suffering mental fatigue (in YOUR opinion), then I would personally voice my concerns with that.
If everything goes on and things do progress to a sexual relationship. Prepare for the backlash, no just from your friend, but also your husband. Your relationship may be sturdy now but under the stress of the long trip to work, having to be her knight, the development of a sexual relationship, his lack of personal time to focus on himself.
All of those things for me would suggest that a sexual relationship is a bad idea. Though I am simply an outsider of course and I can't know specific details or get all sides of the story.
Your friend should be able to make her own mistakes even if she is in a fragile mindset? I personally found that a little indifferent to the relationship you have with her. You should also be concerned with the effect this will have upon her as practically sisters even more so.
This was just an opinion of a probably inexperienced person but I hope things work out well.
Female - 24 - poly