Is this a weak soft mutation? According to other sentences in this section "llygaid" does not lenite in this situation. Also, there's a similar phrase in which "gwallt" doesn't drop the g, so I'm a little confused.
As with ei lygaid e. Possibly the similar phrase you're thinking of would be ei gwallt hi, or simply ei gwallt? The soft mutation here only follows "his"; "her" causes an aspirate mutation.