I'm gobsmacked by this thread, to be honest although perhaps the property situation in Dublin is a factor in this.
Are the daughters in question so precious and lacking in any streetwise-ness that they can't just pay rent [they can work part time, you know, a lot of students do it] and maybe move into an apartment of their own. Maybe it's a cultural thing, maybe it's different here in Dublin compared to wherever these girls may go to college, but while it can be a trial, there is generally plenty of rented accomodation to go around eventually. Yes, you may have to live in a place that isn't perfect, but it's still a formative experience that will help your personal development. In my first year in college I lived in halls of residence or whatever you want to call it, we call it campus here.
For the following two I moved into the centre of town and it was the best thing I ever did - if an apartment had been bought for me at that stage, well, I would expect everything in life to fall into my lap.
If the first solution of parent B was to go ahead and just buy a flat, then I feel that the recipients will have a deluded sense of reality and believe that life is so simple that when there's a little bit of inconvenience, a solution can easily be bought.
This is veering from the point somewhat.
My general point would be that no matter how flimsy the reasons of Parent A may be, Parent B should not have gone over their head and bought a two bedroomed house. That's just not cricket at all, and I would have very little sympathy for such actions. Unfortunately, they may pay for that, but that's the price you sometimes pay for rash decisions. In reality, they should just charge the daughter of Parent A rent to live in the house and treat them as a tenant.
As a general rule, having been through this recently enough, living with close friends can sometimes have damaging implications. Whether you're buying, sharing, or whatever it is, there is always someone who is in the position whereby they are the organiser and are left waiting for rent, bills etc from the others. Sometimes it's late, but because you are such good friends, well, these things can be taken for granted to the point where it causes strain. When money comes into it, then it causes tension that can complicate a friendship. In the case above, I feel that parents getting involved in things makes it twenty times worse.
Crucially, both girls in question need to grow up and take a hold of their own lives.
