According to the official guidance, current visas will be extended to 31 May 2020. In case your wife needs that extension, she has to apply for it. If she stays longer than the original intention but her visa doesn’t expire, she must be perfectly fine.
If you’re in the UK and your leave expires between 24 January 2020 and 31 May 2020
Your visa will be extended to 31 May 2020 if you cannot leave the UK
because of travel restrictions or self-isolation related to
coronavirus (COVID-19).You must contact the Coronavirus Immigration Team (CIT) to update your
records if your visa is expiring.
EDIT: The official guidance states a further extension to 31 July 2020.
If you’re in the UK and your leave expires between 24 January 2020 and 31 July 2020
Your visa will be extended to 31 July 2020 if you cannot leave the UK
because of travel restrictions or self-isolation related to
coronavirus (COVID-19).You must request an extension by updating your records with the
Coronavirus Immigration Team (CIT) if your visa is expiring and you
cannot leave the UK at present but are not planning to stay in the UK
in the long term.If you’ve already had your visa extended to 31 May 2020, your visa
will be extended automatically to 31 July 2020.You are expected to take all reasonable steps to leave the UK before
this date where it is possible to do so.
Normally, the UK authorities take a dim view of significant changes because they consider that someone who applies for a short visit but then stays very much longer is probably working illegally.
However, these are extraordinary times, and if your wife finds herself stuck because of travel restrictions she should have no problem if she explains that in future visa applications.
Note, however, that if she finds that she needs to stay beyond her visa she must contact the authorities to make arrangements for a longer stay.
FWIW, the countries that have imposed travel bans (there’s a list on the NY Times) have not applied them to their own citizens. Your wife is likely to be allowed to travel, but might find it impossible to do so because flights are not available.
Credit:stackoverflow.com‘