Although many people following vegetarian diets (vegan, ovo, ovo-lacto,
etc.) are very conscientious about their nutritional intake, it may be
that the client in question has anemia.  According to the '97
Breastfeeding Answer Book (BAB), "anemia is a common contributor to
repeated mastitis.  Some docs suggest taking supplemental vitamins and
iron or, perhaps, extra vitamin C to combat infections."  Non-heme food
sources of iron are not necessarily well-absorbed and I'd probably
explore this diet connection first.  If the mom is ovo/ovo-lacto, is her
consumption of saturated fats excessive (I doubt it, but just another
diet avenue to explore)?  How is her sodium intake?  Both excessive
amounts and insufficient intake can contribute to recurring mastitis.
Since we don't know how long mom has been following her current diet,
you may want to look at possible food allergens...BAB references this as
well.

Just my $0.01 worth about diet...dietitians notoriously under-charge for
their advice, so maybe one of these days my input will be worth $0.02
:-).  Meanwhile, I'd probably focus  on all the standard causes like
stress, fatigue, etc.