Sometimes people who like historicals prefer books set in "their" time periods and aren't so much interested in stuff that happens in other places and times. I don't know if this would apply to your mom or not, but it's something maybe to keep in mind.
Going by Gabaldon and Gregory, it looks like she might possibly favour British Isles stuff from medieval through Renaissance UK and might be a little more into plotty-lots-of-political/personal-maneuvering-in-detail with maybe a bit more focus on the historical personalities than straight get-together-then-happily-ever-after type romances.
If that's so, I second the recommendation for
Alison Weir. I haven't read her fiction yet, but her non-fiction English history works were informative and entertaining. And while I've only read
Roberta Gellis' fantasy crossover stuff, her writing seemed pretty solid, and you can get a couple of her books free via the
This Sceptre'd Isle Baen promotional CD over at the Fifth Imperium website.
On the Irish side,
Morgan Llywelyn writes marvelous Celtic historicals, often with a fantasy element. The ones I know of in e-book format include
The Lion of Ireland, about
Brian Boru, and
Grania, about the real-life pirate queen
Grace O'Malley who lived during the Elizabeth Age.
While not historical
romances, per se, both have fairly strong romantic threads underlying several of the character relationships, so they might be books that appeal.
Also,
Susan Higginbotham's historical romance,
The Traitor's Wife, set during the reign of Edward II and featuring his niece as the heroine, was recently offered free in all the stores. It won some sort of minor award, according to Wikipedia.
So possibly if you're an inveterate freebie-getter like I am, you may already have this.
Finally, if early US history turns out to be okay, I give a very high recommendation to
Barbara Hambly's two novels about various First Ladies:
Patriot Hearts, exploring the relationships between Martha Washington, Abigail Adams, Dolly Madison, Sally Hemings and their respective Presidents
The Emancipator's Wife, about Mary Todd Lincoln, which was a finalist for the Michael Shaara Award for Excellence in a Civil War Novel. This one's a somewhat weightier and less "romantic" read, but still has a solid and well-depicted love story contained within.