Just think of the subtenant cum owner as two separate people with two separate roles.
Tenant = T
Subtenant = ST
Old owner = OO
New Owner = NO
The rights of the tenant vis-avis the new owner will be informed by the lease and the local laws. Generally, if the sale happens in the middle of the existing lease, the NO is obligated by the terms, as is the T. NO cannot just kick T out, and T cannot just break the lease. The lease may say what could happen and local laws will apply.
Likewise, ST has whatever contract with T that previously existed. Pretend NO and ST are different people. If T owes NO $1000 per month, and ST owes T $400 a month, that continues even though ST and NO are the same person.
- Depends on what kind of dispute.
- See above. All contracts continue, subject to whatever the lease with the original owner and the subtenancy agreement say abut modifying or breaking the lease and sublease. Local laws apply. Note, if the subtenancy was "off the books" or was done when not allowed by the original owner, and if it was not in a place that the local law says owners cannot deny subleasing, then tenant might not have any protection. ST, now that he is owner could just move out and stop paying. On the other hand, even if he is the owner, he cannot just say "I am the owner now, so I am moving back in for free" because the original lease gave the tenant use of the whole property. But ST could just drop out because T always owed OO, and now NO, the full rent.
- Do you mean if rather than sublease, they were both on the original lease? Interesting, but just imagine it as the obligations before the sale = the obligations after the sale. I don't know though. There are some tax implications for an owner occupied rental.