As long as the mucus is not green, which indicates infection, I think that is normal. Is he on antibiotics?
He is taking an antibiotic and an expectorant and I assumed that the change was thining his mucus, but when I was looking up frothy mucus it came up as a symptom of pulmonary edema.
This could mean that the mucus is thinning out, which is a good thing. The thicker the mucus, the harder it is to expell. Is she on an antibiotic and an expectorant? If so, this is the work of the medication.