Leasehold ownership · HM Land Registry

Hello, im posting this on behalf of a family member.

My family member inherited a leasehold property,to be jointly owned with their sibling, after their parents died many years ago.both siblings were beneficiaries and owners with a 50/50 split. neither sibling lived in the property they inherited it and let it out as a rental property.
My family member has only recently found out his sibling extended the leasehold, without his knowledge. His name is not on the new title register with the extended years and there is no restriction on the new title register stating my family member is still a beneficiary with a 50/50 split in the property.
we have only ordered online copies of the original leasehold title register with both their names, the freehold title register and the new updated title register with just his siblings name.

can we order copies of the new leasehold which state the cost for extending the leasehold, and how can we do so?
can we also get a restriction in place on the new leasehold stating my family member is still a beneficiary to maintain his rights to the rental income and any proceeds of sale in the future, would this have to be done via a legal professional?

what documents can we order now to confirm my family member inherited the property as a beneficiary and joint owner would official copies of the 3 title register be enough, old/new(leasehold) and freehold title registers, we are slightly concerned the property has been taken from him without his knowledge and want to gather all the information necessary.