The poor girl. I think Ayumu would be kind to her and have sympathy at least, but as a friend. If anything, I think he'll call of the wedding for her own good and even try to uplift her spirits as a friend. Unlike his dad, Ayumu sees women as people and he's sensitive towards their feelings.
This is a political marriage unfortunately. Both Ayumu and his soon-to-be bride (too lazy to look for her name!) have a public role to play, and they are both perfectly aware of that.
The real issue is, the young girl is probably expected to give an heir to Ayumu ASAP. I guess that is the part they both are really uncomfortable with.
I get that entirely and all, but the thing is that Ayumu still has a lot of the power and is not the type to abuse it. If anything, I picture this taking a unique turn in which all three of them will work together to sort out this political mess.
The real issue is, the young girl is probably expected to give an heir to Ayumu ASAP. I guess that is the part they both are really uncomfortable with.
Just think of the merriest day in your life. No, not that day.