Hi,
As the shares are jointly owned, they both need to calculate their capital gains liability separately. Gains should be reported using the realtime capital gains service at:
Report and pay your Capital Gains Tax
or on a self assessment tax return if they do not use the service before 31 December after the tax year has ended. Capital gains tax need only be reported in a self assessment tax return, if the following two statements are both 'true'. 1 - Do you need to complete a self assessment tax return for any other reason? and 2 - have you disposed of assets with a value over £50000.
Thank you.