Ford expedition, is it my transmission, torque converter, or something else?

2003 ford expedition 4.6 liter triton engine, good engine I
heard. Took it to mechanic because it was difficult to drive
at startup, I paid $950 for new radiator and one of the lines
that connect to radiator was leaking transmission fluid and
fluid was empty so he fixed all that and filled transmission
fluid. He told me hopefully it wasn't running like that for
long or it's going to be the transmission. The next day I was
driving, I should've stayed close to home in case anything
went wrong but my daughter had doctor's appointment,
and drove about 20 minutes and at red light I was able to
go just a little bit and it died. Luckily, I was on hill and was
able to coast down road into gas station. Then I called
mechanic and he had me look at the fluid and was clean.
Then tried to start it up a bunch of times and he said it was
transmission. So now it's in shop and he's putting in a
new(used 70,000 miles) transmission, fixing exhaust leak,
oil change, and inspection. It's costing around $1800
unless he finds something wrong when he does the
exhaust leak fix and could cost more. I heard it could be the
torque converter but not sure, I'm no mechanic. It wasn't
making any unusual sounds, just the regular engine
sounds. This mechanic is our friends dad so I would hope
he's not trying to rip us off or anything but I do know new
transmissions are expensive. I know there will be people
here who say it's not worth it, just buy a new vehicle but I
don't want to buy someone else's problem.