Friend has a Transmission problem

I'm posting for a friend who's son has a Mazda 3 manual transmission. Hoping you can help me out. He started having problems with the trans not staying in gear. When he took it to a mechanic, he found a crack in the housing and his clutch needed replacing. Now what would cause a cracked housing? Heavy clutch usage by "dropping" the clutch hard?
He was given the option of
1) Getting a new housing for the trans and just swap housing
2) A rebuilt Trans
3) A used trans
He was given an estimate of $2000 for the trans work. This would be rebuilding it. Is this reasonable?
A more pressing question is" Is this a common Problem?
Thanks

Where is the crack?

Good question. I will ask him in the morning when I see his dad.
Question: Are the trannies strong so this would be unusual? Or is it a weak trans in these cars? I'll find out more about the mileage, etc. Its a 2005

yeah depending on were the crack is, then ull know what to do with it. im actually working im my tranny.
Question: im having a problem with my clutch master cylinder not engaging my slave cylinder and throwout bearing. its a 2001 mazda b2300 truck.