I think this is a Fistula, which is just a tract from one site to another .
In your Mums case it goes from somewhere in her neck to the outside skin. But fistulas can be anywhere.
Personally, I would not be doing HBO while she still has active cancer. Do discuss this with her oncologist before embarking on this course of action.
Thinking of you both,
Tammy

Caregiver/advocate to Husband Kris age 59@ diagnosis
DX Dec '10 SCC BOT T4aN2bM0 HPV+ve.Cisplatin x3 35 IMRT.
PET 6/11 clear.
R) level 2-4 neck dissection 8/1/11 to remove residual node - necrotic with NED
Feb '12 Ca back.. 3/8/12 total glossectomy/laryngectomy/bilat neck dissection/partial pharyngectomy etc. clear margins. All nodes negative for disease. PEG in.
March 2017 - 5 years disease free. Woohoo!