The pressure in wet concrete is sneaky and hard to control if your
form isn't right. The best way to make the form would be to put a
1x6x18" board horizontally on both sides of the wall, then on the
outside of each 1x6, put a 2x2 upright about halfway along the
opening, then get some thick wire (#9 if available) and wrap it around
the far 2x2, over the boards, then on the near 2x2, twist the wire
together tightly, trying to compress the form together if possible. A
bar clamp would be a big help squeezing the 2x2's together while you
twist the wire tight. Fill in the opening with more 1x6 boards placed
horizontally, and every second board on the way up, put in another
piece of #9 wire to hold it all together. The first pour could be
done up to about half way up, the second pour could be done up to 6"
from the top. The very last should be done with a very dry mix,
pressed into place with a small bricklayer's trowel (blade about 5"
long). If this last bit tends to slump out of place, just leave it
for a few hours, then continue. When its basically full, you could
put a layer of mortar over the whole last part, then a few hours
later, using a straight edge and the bricklayer's trowel, shave the
surface down to a nice flat surface. The day after the pour, you can
remove the forms, but don't disturb the wires much for a week.
Concrete achieves 70% of its ultimate strength in a week, so this time
is needed to make a good seal around the wires.
I think the way I would fill in the opening would be to use
concrete blocks as the form to hold the concrete. You can cut block
with an abrasive blade on a skilsaw. Assuming your wall is 8" thick,
cut 5" off each end of an 8" concrete block. You might have to break
these 5" "U" pieces in half to get them in place around the plumbing
drain. Put mortar on the top edge of the previous block, then set the
next piece in place, tapping lightly with a hammer. Hold a straight
edge across your existing wall to see that each new piece is lining
up. When a few courses are hardened into place, fill the voids with
mortar or concrete. Use a jointer tool and your bricklayer's trowel
to fill in any gaps, then about 15 minutes later, use the jointer to
make the decorative groove typical of blockwork.
Tell your wife that I think she's got herself a good man.