I am currently NOMAIL for the past few days, and am obviously still strying
to catch up even from last week, so bear with me if this has been covered:
 As a long time vegetarian Mom and nurser, in the absence of any real
indication I don;t see a connection between a mother's vegetarian diet which
has sustained and grown her baby well for six months and  sudden
 constipation in her baby. As far as the literature is concerned the idea of
"complementary" proteins, if by this you mean food combining,at each meal, is
out of date.  What specific knowledge does Mom seem to be lacking ? Does she
simply remove animal protein from her diet and then not add any legumes,
fortified soymilk etc.?   Currently vegetarians and vegans do what moms of
toddlers do, look at protein intake over days, not necessarily at each meal,
although the complementary thing at each meal used to be standard thinking.
It seems that, as you said this baby, may simply not be ready for solids. If
six months is average then some will start at 4 and some at 8. A subscription
to Vegetarian Times magazine might give some recipe ideas. If you would like,
I could repost again some good Veggie cookbooks. Judy Fram, Brooklyn, NY