Just because she got an abcess is no reason to suddenly convert a barefoot horse to shod. These things just happen, could happen to a shod horse as well, probably morse so since there are nail holes for moisture to get in and breed bacteria.
Abscesses can be scary, and they can occur to shod horses just as easily as barefoot ones, so don't let this scare you into shoeing. In fact, I see more abscesses blowing out of horses coming out of shoes than I do in already bare feet!
And ANY horse can get an abscess. My old guy stepped on a nail and had a horrible abscess this spring. Most of the sole is gone from the toe now, but he's quite happy. I just slap a boot on if he's going to go over rocks, since the sole is missing protection of it's own, the boot does the job. But as scary as it looked and still looks, it's a pretty minor thing in my book.
Epsoms salt soaks, apple cider vinegar soaks, both work well. Itchamol can help draw it out.
Once the hole opens up, put iodine soaked cotton balls in it, wrap with vetwrap and duct tape. Change daily for a few days, then don't worry about it. Unless the abscess was CUT open, by the time it opens on it's own, the body will have sealed the pocket of nasty off from the blood stream. CUTTING it open is not recommended, as the body won't have sealed the bloodstream off yet, so you could actually make the abscess worse and take longer to heal than just letting it blow on it's own.
Either way, it's still not a huge deal to worry too much over.