There are perhaps two factors here.  It definitely sounds like oversupply.
The fact that he does better with expressed breastmilk is one clue. "Night
is better." Is she nursing lying down?  That is one recommendation, as the
over supply can be better handled by the baby.  He can control what goes
down his throat by letting excess run out of his mouth. I sometimes have to
recommend one breast for longer than 4 hours.  She is concerned that he
isn't getting enough now.  Ask about wet diapers.  He may have decreased
from 15 to "only" 8 a day.  And he is perhaps still crying.  An oversupply
has to be decreased a lot for it to help.
"Nursed well for 3-4 weeks." When did she begin diet drinks?  Perhaps she
has begun dieting and increased the drinks.  I have found that in most
babies even one diet drink or other aspertame use can cause severe cramping
in the baby's intestines.  One mother said she drank enough diet soda to
float battleships.  She didn't think she could go without it.  But she tried
for a few days and had such a "wonderful" baby that she found the reward to
be worth the sacrifice.  I suggested that while she was experimenting she
get some juices that she felt were really great or that she drink regular
(caffeine-free)sodas for that time. This helped with "withdrawal". Diet
sodas are so revered that some mothers are "ashamed" to order a regular one
in public.  It looks non-nutritious. (!)

Also, ask when she eats the dairy foods. Since the crying seems to be
somewhat time of day related.

Check for any other changes in her diet since the 4-week period, including
food, drink, meds.,anything by mouth.

Pat Gima, IBCLC
Milwaukee