The only way I can see how she'd be offended is if she somehow thought you were calling her fat by tagging her in the photo of the prego mare??? Whatever the reason, unfriending her was probably the best decision. If she wants to add you again, I would limit anything she can see. If it's a diss being unfriended, it's a bigger dissbeing refriended and not able to see anything more than if you weren't friends with the person.
On the note of putting her in the barn, I would play it by ear. Since you're in Ontario, snow is definitely a factor and it isn't unheard of for anywhere in Canada to get a freak snowstorm between March and May. That being said, you may want to accustom her to being in the barn sooner rather than later should you find that the weather neccesitates you to have her there when she's near due. If it becomes a routine, she won't feel as much anxiety.
When my mare was in foal, we didn't have a large pasture on the yard where she'd be able to go off and pick her spot away from the others and our larger pasture was too far way from the yard (1/4 mile away) that I was comfortable leaving my mare out there to foal. We did separate her from the herd about 3-4 weeks prior to her due date. To help ease some of the anxiety, I made sure I spent as much time with her as possible, grooming etc., especially the first couple of days. After that, she settled right in and didn't mind being solo. Of course I made sure I went out daily to groom, give her treats and lovings, etc. and she had no issues foaling outside. That was late June though, so there wasn't a risk of snow or cold temperatures.