Thanks for getting back to me!
The fact that the boil appears to be softening is a favorable sign, as it indicates that fluid may be forming that can be drained. Although her play habits are not conducive to creating a rapid healing environment, I see no reason as to why this should make the boil worsen. If anything, it may help to encourage softening of the swelling within the boil. Also, as she has not had a large boil while in your possession, this also increases the chance that it can be resolved.
So to answer your question, yes, there are treatments that can be tried. The cold water poultice is usually attempted first, along with DMSO, which is a potent topical anti-inflammatory. With the size of her boil, however, chances of it reducing with just poulticing are low. I would recommend perhaps having another vet take a look if there is one in your area. By inserting a needle into the boil, it will be able to be determined if there is a major fluid pocket underneath the skin that can be drained. If so, the boil can be drained using either a large gauge needle or a small stab incision. Following drainage, a pressure bandage needs to be applied to help prevent the fluid from re-forming. There is the challenge, especially as Riptide is so active in her paddock. I would recommend using a tape called Elastikon, which is a stretchable adhesive tape with excellent adhesive characteristics. I have seen people get very creative in their bandaging! When treating knee boils (carpal), I will often inject a steroid directly into the area which was just drained. This can help to knock out any residual inflammation, which is the major source of the fluid collection. Some practitioners also recommend the insertion of rubber tubing through the boil while it is bandaged, which can help to keep the drain hole open, preventing fluid build-up while the incision heals. As a last resort, surgical resection of the boil is attempted, which entails opening the boil and removing any residual scar tissue and excess skin, then bandaging for a period of 10-14 days while the site heals.
So yes, you do have options. I hope you can find a vet that will work with you, and I'll be sending you good thoughts that all will be resolved. I've included two links for you that discuss this condition in further detail. If you are unable to read the first link, I recommend registering for the site, as it is free, and contains much useful information concerning horse health.
Please let me know if you have any more questions.